since i.school,I haven't had much spare time.A.begun.B.began

B.
这道题要注意,since的翻译.since可以做时间状语翻译为“自从某时开始”,还可以做原因状语翻译为“由于,因为”.
因为后面用的句子是完成时“I haven't had much spare time”,所以这个since应该是时间状语翻译为“自从”,在这种用法时,他后面接的时间状语要是过去的某一个时间点,所以用过去时began.