QUES:
Who is they in the following sentence?

Buy More Book Store sends brand new books to customers, but they sometimes end up sending used books instead.

ANS:
Buy More Book Store

QUES:
Who is they in the following sentence?

The security patrol searched the land for survivors, but they were not interested in saving them.

ANS:
The security patrol

QUES:
Who is she in the following sentence?

Mary lied to Sue and she slapped her.

ANS:
Sue