Mum: Get up, Mike!

Mike: Mum, what time is it now?

Mum: It's eight o'clock.

Mike: Oh, it's time to go to school. I'm late.

Mum: Don't worry! It's Saturday today.

Mike: I forgot it（忘记）. What's the weather like today?

Mum: It's rainy.

Mike: Can I go to the library?

Mum: No, you can't. It's cold outside. It's only 2 degrees. You can read books at home.

Mike: OK, mum. Where's my schoolbag?

Mum: It's in your study.

Mike: Thank you, mum!

抛物线y=x²-2$\sqrt{a}$x+a²的顶点在直线y=2上，则a的值为（　　）