Jack London was the author of "To Build A Fire"
It would be the girls’ dogs since there are multiple owners of the dogs. You keep the girls plural and use the apostrophe at the end to make it girls’ since it is both plural and possessive.
Answer:
a. on
b. I don't think here a preposition is needed as "This is the key to my room" is a complete sentence.
c. in
d with
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