Tourism, _______ it brings in money to a country, can also result in damage to the World Heritage sites there．
| A．once | B．while | C．if | D．as |
She will tell us why she feels so strongly that each of us has a role _____ in making the earth a better place to live.
| A．to have played | B．played | C．playing | D．to play |
The Russian rocket designer has died. For many years, he ______ as an important assistant to the father of Soviet space program.
| A．serves | B．was serving | C．served | D．has served |
I have been living in the United States for 20 years, but seldom ______ so lonely as now.
| A．have I felt | B．I had felt | C．I have felt | D．had I felt |
—How was your trip to London?
—Great. I was helped by an organization. Otherwise, I ______ it.
| A．wouldn’t afford | B．hadn’t afforded |
| C．wouldn’t have afforded | D．needn’t have afforded |
Our coach says we won’t keep winning games ______ we keep playing well.
| A．because | B．unless | C．when | D．while |