单个流速测点上的流速历时一般采用100 秒。当流速变率较大或垂线上测点较多时，可采用（）秒。
A、30
B、60
C、100
D、30-60
参考答案：D
His parents are very concerned about his son at school because he _______moving in his seat.
A．is always keeping
B．always keeps
C．does keep
D．has always kept
2米4厘米写成小数是______米，写成整数是______厘米．
