It is generally considered that air pollution is ______ danger to ______ nature.
| A．/; a | B．/; the | C．/; / | D．a; / |
27．If you had worked hard, you ______ so anxious now．
| A．are not | B．would not have been |
| C．will not be | D．would not be |
26．______ I'm willing to help, I do not have much time available．
| A．While | B．When | C．Once | D．Since |
25．The rise of oil prices made scientists search for new energy resources ______ oil．
| A．replace | B．replacing | C．to replace | D．Replaced |
24．You didn't believe John won the first prize at the speech competition, ______?
| A．did he | B．didn't he | C．did you | D．didn't you |
23．I had reached a point in my career _____ I needed to decide which way to go．
| A．why | B．which | C．when | D．Where |