Last year Luis invested x dollars for one year, half at 8 percent simple annual interest and the other half at 12 percent simple annual interest. Now he wants to reinvest the x dollars for one year in the same two types of investments, but the lower rate has decreased, If the higher rate is unchanged, what fraction of the x dollars must be reinvest at the 12 percent rate s that the total interest earned from the x dollars will be the same for both years ？（）
(1)The lower rate is now 6 percent.
(2)The total amount of interest earned from the two investments last year was $3,000.
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：A
解析：
根据(1)可知较低的年单利率为6%，设总投资额为x，12%的年单利率的投资所占的百分比为n，则得到如下等式：
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由上面的方程，消去x，可以解得n的值，所以(1)能充分地回答上面的问题；根据(2)无法求出年单利为12%的投资占总投资的百分比。