Two children, Bob and Mary, have piggy banks into which they deposit money earned from doing odd jobs. In a certain year, both Bob and Mary each deposited $5 on the first of every month into their respective piggy banks. If these were the only deposits made into the piggy banks during the year, on December 31 does Bob have more money in his piggy bank than Mary has in her piggy bank (Assume no withdrawals)（）
(1)On March 15 Bob had three times as much money in his piggy bank as Mary had in hers.
(2)On June 15 Bob had twice as much money in his piggy bank as Mary had in hers.
A．条件(1)充分，但条件(2)不充分．
B．条件(2)充分，但条件(1)不充分．
C．条件(1)和(2)单独都不充分，但条件(1)和条件(2)联合起来充分．
D．条件(1)充分，条件(2)也充分．
E．条件(1)和条件(2)单独都不充分，条件(1)和条件(2)联合起来也不充分．
参考答案：D
解析：
因为两个人都在每个月的同一时间向各自的扑满中加入相同数目的钱，所以不管在哪一个月的几号，只要一个人扑满中的钱比另一个人的多，则在年未12月31号时，他的钱都一定多。由以上分析可知(1)和(2)单独都能充分说明在12月31号时，Bob扑满中的钱比Mary扑满中的钱多。