Some people argue that using _____ nature to meet our own needs may spell disaster, unless we start to look around and understand how ______ clean environment benefits us all.
| A．/; a | B．the; / |
| C．the; a | D．a; a |
By the time you ____ in, I ____ two letters.
| A．had come, had written | B．came, wrote |
| C．come, have written | D．came, had written |
To ____ your dream of becoming a scientist, you must work hard from now on.
| A．turn | B．recognize | C．live | D．change |
I ____ go to bed ____ the telephone rang.
| A．was about to, when | B．was going to, while |
| C．would, when | D．was to, while |
You may ask the teacher for help ____ difficulty.
| A．in case of | B．in the case of | C．in any case | D．in no case |
The train ____ at present speed until it reaches Beijing.
| A．is running | B．will be running | C．will have run | D．has run |