天然气的主要成分是烃类气体。
参考答案:对
—Could you tell me what time the train leaves Hefei for Beijing?
—I'm afraid you need to _____ on the Internet. [ ]
A. look for it
B. look at it
C. look it over
D. look it up
A.频率越低,波速越大
B.频率越低,波长越长
C.波长越长,波速越大
D.电磁波的波长都相等