The country has already sent up three unmanned spacecraft, the most recent _____ at the end of last March.

A．has been launched

B．having been launched

C．being launched

D．to be launched

“先将总体的观察单位按一定的顺序分成若干部分，再按照一定的顺序，每间隔一定数量的单位抽取一个单位进入样本。”此种抽样方法属于（）

A.随机抽样

B.系统抽样

C.分层抽样

D.整群抽样

E.方便抽样