15.椭圆C:x2a2+y2b2=1(a>b>0)C:\frac{x^2}{a^2}+\frac{y^2}{b^2}=1(a>b>0)C:a2x2+b2y2=1(a>b>0)的左顶点为AAA,点PPP,QQQ均在CCC上,且关于yyy轴对称.若直线APAPAP,AQAQAQ的斜率之积为13\frac{1}{3}31,则CCC的离心率为((( )))