下蹲控制姿势不变股四头肌的收缩形式是（）
A.等长收缩
B.等张收缩
C.离心收缩
D.向心性收缩
E.等动收缩
参考答案：C
圆的周长和直径的______叫做圆周率．
How are you going on with your English?（）
A.I am very well.
B.I am going to the library.
C.I am going to take College English Test Band Four.
D.That's a good question
