地球上最早的生命出现在[ ]
A．原始大气
B．原始海洋
C．陆地上
D．大气层中
答案：B
He has read many books on science, so it's easy for him _____ these questions.[ ]
A. answers
B. to answer
C. answering
D. answer
空气预热器分几种型式？
