具有径向泵的液压调速系统，当转速的相对变化量为2%时，调速器油压的相对变化量为（）。
A.1%
B.2%
C.3%
D.4%
参考答案：D
______是地球上最大的生态系统；在生态系统中，细菌和真菌被称为______者．
The one concern for parents and students is _______ the student pursues their university education in a foreign country, the student will not have the opportunity to establish social contacts in China.
A．that
B．that if
C．whether
D．how
