Question 1
The car alarm _____ went off ten minutes ago is still on.
We use which (or that) for things. Here it refers to the car alarm and is the subject of went off. Who is only for people.
Question 2
The Moon, _____ diameter is only a fourth of the Earth's, is the only natural satellite of the Earth.
Whose shows possession and is followed by a noun: the Moon's diameter → whose diameter. It works for things as well as people. That cannot be used for possession or after a comma.
Question 3
The burglar _____ no one saw entering the apartment stole jewellery worth a hundred thousand dollars.
Whom refers to a person and is the object of the clause: no one saw him → whom no one saw. Which is for things, not people.
Question 4
He panicked when he couldn't find the key in his pocket _____ he always kept it.
Where introduces a relative clause about a place: his pocket is the place in which he kept the key. Which would need in: the pocket in which he kept it.
Question 5
We hired a new engineer _____ worked for a big electronics company for years.
Who is used for people and is the subject of the clause: the engineer worked for a big company. Which is for things, and whose needs a noun after it.
Question 6
Newton made his discoveries during a time _____ humans didn't know much about science.
When introduces a relative clause about a time: a time when humans didn't know much. On which is wrong; with time we would say at which or in which.
Question 7
Cities _____ history goes back thousands of years usually have a lot of historical sites.
Whose shows possession: the cities' history → cities whose history. Of which would need a different word order: cities the history of which.
Question 8
It's important that the groceries _____ we buy every day do not contain any harmful chemicals.
When the relative pronoun is the object of the clause (we buy the groceries), we can leave it out: the groceries we buy. Which or that is also possible, but no option needs to be added here.
Question 9
Scientists are still trying to find the reasons _____ tornados and hurricanes form.
After the reason(s) we use why: the reasons why tornados form. When is for times and where is for places.
Question 10
The person _____ she was waiting turned out to be a close friend of mine.
The verb is wait for, so the preposition is needed: she was waiting for the person → the person for whom she was waiting. Not whom alone, because then for is missing.
Question 11
In primary school, I had some very good friends _____ names I still remember.
Whose shows possession and is followed by a noun: my friends' names → friends whose names. Of whom cannot be used directly before names.
Question 12
Football is one of the things _____ he has always been very keen on.
The pronoun is the object of keen on (he is keen on the things), so it can be left out: the things he is keen on. Not to which, because the preposition with keen is on, and it is already at the end.
Question 13
On the day of the picnic, it started to rain in the morning, _____ was a big disappointment.
After a comma, which can refer to the whole previous idea: the rain starting was a disappointment. That cannot be used after a comma in this way.
Question 14
The skirt _____ colour I didn't like that much actually fitted me better than the other.
Whose shows possession and is followed by a noun: the skirt's colour → the skirt whose colour. That cannot show possession.
Question 15
Maths, _____ most students aren't very fond of, is my favourite subject.
Which is used for things in a clause between commas: most students aren't fond of maths → Maths, which most students aren't fond of. The preposition of is already at the end, so at which is wrong.
Question 16
The necklace _____ she always wears for special occasions is from her grandmother.
That (or which) is used for things and is the object here: she always wears the necklace. Whom is only for people.
Question 17
I can only recommend the holiday resort _____ we spent a week at this year.
Which refers to the resort and goes with the preposition at at the end: we spent a week at the resort. Not where, because where already means at which, so at would be repeated.
Question 18
I sometimes see some famous people having a cup of coffee at the cafe _____ is at the corner of my street.
Which is the subject of is: the cafe is at the corner. Not where, because where cannot be the subject of a verb; it needs a subject after it (the cafe where we meet).
Question 19
The small orchard near the church _____ we used to eat cherries and apricots doesn't exist anymore.
Where refers to a place: we used to eat cherries in the orchard. Which would need a preposition, e.g. in which.
Question 20
Liv and Tyler met each other in 1994, _____ they went to an ancient village _____ they carried out some excavation works.
The first gap refers to a time (in 1994), so we use when. The second refers to a place (an ancient village), so we use where.
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