Помогите, пожалуйста!Choose correct answer:Where their apartment located?а)has b) is c)

Помогите, пожалуйста!
Choose correct answer:
Where their apartment located?
а)has b) is c) been
The letter written by student.
а)are b) were c) was
This book will be . a lot of about.
а)built b) repaired c) spoken
The salad has already cooked.
а)been b) being c)benefit