泵控型半闭式液压系统通常是从主泵（）。
A.低压侧泄油和补油
B.高压侧泄油和补油
C.高压侧泄油，低压侧补油
D.排出侧泄油，吸入侧补油
参考答案：A
如果等差数列{an}中，a3+a5=12，那么a4=[ ]
A．12
B．24
C．6
D．4
Much attention should ______ the environment and fighting against pollution.
A．be paid to protect
B．pay to protecting
C．pay to protect
D．be paid to protecting
