偶函数f（x）对任意实数x，y恒有f（x+y）=f（x）+f（y）-2xy-1，则f（x）的表达式为______．

选择合适的答语。

(     1. What would you like?

( )2. Can I help you?

( )3. What's your father?

( )4. Your school is cool !

( )5. What's in your schoolbag?

A. Thank you.

B. I'd like some ice.

C. Yes, pass me a plate.

D. He is a doctor.

E. An English book.