Once _,the clock will last a month and keep good time.A.start B.startedC.to start D.starting

Once _,the clock will last a month and keep good time.A.start B.startedC.to start D.starting
Once _,the clock will last a month and keep good time.
A.start B.started
C.to start D.starting

Once _,the clock will last a month and keep good time.A.start B.startedC.to start D.starting
B.这是虚拟语气,需要被动语态.意思是：一旦这个钟可以走动,能过持续一个月并且走势准确.

B
钟自己不能走