What did you do last night，Bob？

First I did my homework，and then I played ______ piano for half ______ hour．（　　）

A．the；aB．/；anC．the；anD．a；the

It is necessary ______ us to help our friends out when they meet difficulties．（　　）

A．toB．ofC．withD．for

Hi，Sara．Is this ______ English book？

No．______ is on the desk．（　　）

A．your； MineB．your；MyC．yours；MineD．your；My

Mount Xiaolei is not far away from here，but it will still take us a few hours ______ there by bike．（　　）

A．gettingB．to getC．gotD．get

What were you doing ______ I knocked at the door？

I was sleeping．（　　）

A．unlessB．onceC．whenD．while