- How old are you?
- I'm _____ years old.( )
A:fifth
B:five
C:fourth
下列物质中,属于纯净物的是( )
过双曲线x2a2-y2b2=1a>0,b>0的左焦点F(-c,0)(c>0)作圆x2+y2=a24的切线,切点为E,延长FE交双曲线右支于点P,若OE→=12OF→+OP→,则双曲线的离心率为( )