Man's understanding of nature is developing .It never stays at the sane level.
问题描述：
Man's understanding of nature is developing .It never stays at the sane level.
A.at the right time
B.for the frist time
C.from time to time
D.all the time
答
D.
A.at the right time在一个适当的时刻
B.for the frist time第一次
C.from time to time时不时地
D.all the time一直；向来,一向；时时刻刻；每时每刻
人类对自然的了解一直在发展之中,从未停止在一个水平上.