21．---Shall I tell your best friends what happened to you yesterday?
— they ask you．
| A．Not if | B．Not unless | C．Not except | D．Not though |
It was not until he arrived at the office_____ realized he had forgotten the key. 
| A．and he | B．when he | C．that he | D．he |
. You can’t imagine _______ when they received those nice Christmas presents. 
| A．how they were excited | B．how excited they were |
C．how excited were they | D．they were excited |
. ____ of the land in my hometown ____ covered with trees and grass.
| A．Two-fifths; is | B．Two-fifth; are | C．Two-fifth; is | D．Two-fifths; are |
. It is difficult for us to _________ a conversation with so much noise around us. 
| A．communicate with | B．carry on | C．set up | D．bring  up |
I failed again in the examination and only then _____ how much time I had wasted. 
| A．I realized | B．I had realized | C．did I realize | D．had I realized |