问题
单项选择题
The price per share of stock X increased by 10 percent over the same time period that the price per share of stock Y decreased by 10 percent. The reduced price per share of stock Y was what percent of the original price per share of stock X ？（）
(1)The increased price per share of stock X was equal to the original price per share of stock Y.
(2)The increase in the price per share of stock X was 
the decrease in the price per share of stock Y.
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．
答案
参考答案：D
解析：
设股票X每股的原始价格为x，则由(1)可得股票Y的每股原始价格为(1+10%)x，每股减少的钱为10%(1+10%)x，因此股票Y每股减少的钱与股票X的每股原始价格的比为[*]，所以(1)是充分的；根据(2)可得股票Y每股降的钱为[*]，也即股票Y每股减少的钱是股票X的每股原始价格的11%，所以(2)也是充分的。