如图所示,甲、乙两同学从河中O点出发,分别沿直线游到A点和B点后,立即沿原路线返回到O点,OA、OB分别与水流方向平行和垂直,且OA=OB.若水流速度不变,两人在静水中游速相等,则他们所用时间t甲、t乙的大小关系为( )A.t甲<t乙 B.t甲=t乙 C.t甲>t乙 D.无法确定 答案:C解析:设两人游速为v, 则有t甲=+=,t乙=, 又lOA=lOB, 则=>1, 故t甲>t乙,选项C正确.19.This work needs close teamwork.________will be achieved unless we work well together.( )A.NothingB.AnythingC.SomethingD.Everything