钥匙在书桌上吗？ ( )the keys ( )the desk 它不在椅子下面 It✀s ( ) ( ) the chair

( Are)the keys ( on)the desk?
It's (not ) ( under) the chair.
I think (it's)(in ) your grandparents' room.
或I think (they're ) ( in) your grandparents' room.

如果整个三个句子都是关于keys的，那么第二句也应该为复数，既然您给的句子不是，那我就认为这三句话彼此是独立的。那么第三句用单数或复数都有可能。

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Are the keys on the desk? It's not under the chair.
I think they're in your grandparents' room.
希望能帮到你
补充一下，你头一句的key加了s，后面回答是it's。估计是你多写了s，所以下面的回答更合适。上面的是根据你提供的第一句话写的。
Is the key on the desk? It's not under the chair.
I think it's in your grandparents' room.

Is the key on the writing desk? It's not under the chair.
I think It's in your grandparents' room.

they are