原始社会经历了()和()两个阶段.;
参考答案:原始人群;氏族公社
– You ought to see a doctor about that cough.
-- I guess I should. I _____ it off for days.
A.put
B.was putting
C.have been putting
D.am putting
一空心金属球恰好可悬浮于10摄氏度的水中,当水温从10摄氏度下降到1摄氏度的过程中,该金属球会______.