线损指标完成情况要对完成值与（）进行比较分析。
A.同期值
B.指标值
C.理论值
D.前三者同时
参考答案：D
We should often keep _____ touch with each other, because we are good friends. [ ]
A. on
B. to
C. in
D. up
声波在气体、液体、固体中的传播速度分别是多少？
