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
网友回答
A. has been launched
(答案→)B. having been launched
C. being launched
D. to be launched
解析：考查独立主格结构：句意：这个国家已经发射了三颗无人飞船，最近的在去年三月份被发射。句中没有连接词，故不能选作谓语动词的A项；由at the end of last March可知应选用表示动作完成the most recent having been launched为独立主格结构，选B项。