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Итак, это просто, гляди:
a: Whose camera is this?
b: It's Bob's. It's his camera. It's his.

a: Whose watch is this?
b: It's Tony's. It's her watch.  It's hers.

a: Whose skateboard is this?
b: It's Bill's. It's his skateboard. It's his.

a: Whose football is this?
b: That's Paul's and Anna's. That's their. That theirs.

a: Whose alarm clock is this?
b: It's Pat's. It's his alarm clock. It's his