Can you tell me _____________________?

A．when you did it so well

B．when did you do it so well

C．how you did it so well

D．how did you do it so well

学生春游到公园划船。如果在5条船上每船坐3人，其余的4人坐一船，则有5人无船可乘；如果在4条船上每船坐6人，其余的3人坐一船，则最后空着一条船无人乘。问：共有船多少条( )

A．36

B．9

C．7

D．18