In aerospace devices the weight of the necessary power producing equipment is _____.
| A．vital | B．possible | C．capable | D．reliable |
There is no doubt the earth is becoming warmer.
| A．whether | B．that | C．if | D．why |
Using a telescope we could see that something .
| A．is happening | B．happened | C．would happen | D．was happening |
all day, I went to bed early.
| A．Working hard | B．Worked hard | C．Having worked hard | D．To work hard |
There is a natural phenomenon scientists call the greenhouse effect.
| A．that | B．what | C．for which | D．who |
Every time I returned after an , I felt stupid because I was behind the others.
| A．ambition | B．abandon | C．absence | D．appoint |