题目

在O点有一波源，t=0时刻开始向下振动，形成向左传播的一列横波。t1=4s时，距离O点为1m的A点第一次达到波峰；t2=7s时，距离O点为6m的B点第一次达到波谷。则以下说法正确的是（    ）     A.该横波的波长为2m B.该横波的周期为2s C.距离O点为4m的质点第一次达到波峰的时刻为6s末 D.距离O点为4m的质点第一次开始向上振动的时刻为6s末The Browns have six children altogether. It is obvious that ________ little money the present job brings Mr. Brown can hardly support so large ________family like that. A．/; the            B．a; the             C．a; /              D．the; a