To battle world hunger, the great scientist once traveled to many places where populations _______ faster than crop production at that time.
| A．expanded | B．were expanding |
| C．has expanded | D．would expand |
______ you may have, you should gather your courage to face the challenge.
| A．However a serious problem |
| B．What a serious problem |
| C．However serious a problem |
| D．What serious a problem |
That’s the factory _______we paid a visit last year.
| A．where | B．to which |
| C．which | D．that |
The field research will take Joan and Paul about five months; it will be a long time _____ we meet them again.
| A．after | B．before |
| C．since | D．when |
All the travelers _____that they should bring their ID cards with them.
| A．have reminded | B．are reminded |
| C．were reminding | D．had been reminded |
There is a big dog _______to a fence outside the house.
| A．tying | B．tied |
| C．to tie | D．ties |