A：aren't
B：shall they
C：will they
D：won't they
南北战争是美国历史上的第二次____革命．经过这场战争，美国废除了____，扫清了资本主义发展的又一大障碍，为以后经济的迅速发展创造了条件．
求下列各式的值：sinarcsin（﹣32）．
Many students in our school _______ the old and they usually offer their seats to them on buses.
①形成四分体 ②同源染色体分离 ③交叉互换 ④联会 ⑤染色体复制 ⑥染色单体分离．
