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

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A. has been launched
(答案→)B. having been launched
C. being launched
D. to be launched
解析：
这题考查独立主格结构，结构是：名词+非谓语，此句前后句没连词，只有逗号是不能连接两个句子的,所以后边the most recent      at the end of last March就是独立主格的形式了，在句中做伴随状语，又因为事情发生在过去，所以用现在分词完成时。选B。