What should we do if it _____ tomorrow?
| A．should snow | B．would snow | C．snow | D．will snow |
Mr. Herpin is one of the foreign experts who ___in China.
| A．works | B．is working | C．are working | D．has been working |
Anyone can borrow books from this library _______ he keeps them clean and returns them in time.
| A．even if | B．unless | C．so that | D．as long as |
So fast _____that it is difficult for us to imagine its speed.
| A．light travel | B．is light travel | C．does light travel | D．travels light |
Just as the tree is the unit of a forest and the book the unit of a library, ______ the molecule is the unit of matter.
| A．as | B．like | C．so | D．thus |
The door opened and in ________.
| A．rushed they | B．the children rushed | C．they rushed | D．did the children rush |