原始社会经历了()和()两个阶段.;
参考答案:原始人群;氏族公社
Siberian Tigers ________ and killed at such a speed that they will disappear from the earth in the near future.
A.hunted
B.are hunted
C.are being hunted
D.had been hunted
供暖系统按照热媒的不同可以分为()。
A.热水供暖系统
B.蒸汽供暖系统
C.太阳能采暖系统
D.热风采暖系统