转斗换向阀是一只三位阀，它可控制铲斗前倾、后倾和（）三个动作。
参考答案：保持原位
You _____ be hungry already-you had lunch only two hours ago! [ ]
A. wouldn't
B. can't
C. mustn't
D. needn't
为防止漫溢险情的发生，可采取洪水调度、分滞洪和（）措施。
A.引水
B.防冲护坡
C.反滤排水
D.抢筑子堤
