He had hoped to have an opportunity to travel abroad for long. But it was a(n) ______ that he missed it when it came last month.
A:surprise
B:joy
C:pity
D:ache
已知椭圆x2a2+y2b2=1a>b>0的左、右两焦点分别为F1 F2 ,点A在椭圆上,AF1→·F1F2→=0,∠F1AF2=45°,则椭圆的离心率e等于 ( )
生物最基本的分类单位是( )