B. There might be a safety problem in hydrogen production.
C.They may still be a cause of global warming.
D.They are not suitable for long-distance travel.
答案：C
38.If what is proposed by the two scientists becomes true,___________________.
A.air pollution will，become a thing of the past
B.there will be no need for gasoline substitutes
C. people will be able to use much cheaper engergy
D.there will be no more biofuel-powered vehicles
答案：B
39. Which of the following Call best describe the attitude of Martin I Hoffert to the proposal?
A.Indifferent. B. Positive. C.Suspicious.D.Critical.
答案：B
40. The passage is mainly written to________________。
A. introduce a new concept
B.compare different energy sources
C.stress the importance ofgasoline
