设集合P={x|x²=1}，Q={x|ax=1}，若Q⊆P，则实数a的值所组成的集合是______．

If x dollars are invested at 12 percent for one year and y dollars are invested at 8 percent for one year, the annual income from the 8 percent investment will exceed the annual income from the 12 percent investment by $64. If $5,000 is the total amount invested between x and y, how much is invested at 12 percent ？（）

A. $1,680

B. $1,997

C. $3,003

D. $3,320

E. $3,500