下列各式，正确的是（　　）

长江三峡电站是将水流的____转化为电能。

根据短文理解，选择正确答案。

    I have two pets. They are Fly, a dog, and Huahua, a cat.

    Fly is brown and grey. He is naughty(淘气的). He sometimes steals(偷) food in the kitchen.  Once, when there was nobody at home, Fly pulled (拉) a piece of meat off the table. But he was not careful enough. The plate also fell to the floor. When I got home, I saw the broken plate all over the floor, so I became very angry. Fly was so afraid that he went back to his bed at once.

    Huahua is a white and yellow cat. She is not beautiful but she is very lovely. Every night at weekends, Huahua sleeps on my bed. When she sleeps, she always snores. It is very funny.

    Huahua is also naughty. Whenever she does something bad and my grandma tries to catch her, she runs away quickly and hides quietly under the bed.

    Fly and Huahua love to play games with me and I like them very much.

如图，点A在半径为3的⊙O内，OA=$\sqrt{3}$，P为⊙O上一点，当∠OPA取最大值时，PA的长等于（   ）.