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
