龄级
参考答案：
按一定的年龄间隔划分的年龄等级。
富铝土广泛分布在世界上的热带和（）。
I ________ use a clock to wake me up because at six o'clock each morning the train comes by my house.[ ]
A. couldn't
B. mustn't
C. shouldn't
D. needn't
