临床上年轻恒牙异常松动的原因多为（）
A.牙根未发育完全
B.根尖周病
C.牙髓炎
D.牙龈炎
E.外伤
参考答案：B
下列比例尺中最小的是（ ）
A．1厘米代表实地距离30千米
B．1：30000000
C．三十万分之一
D．
I _____ early and _____ an hour's walk before breakfast.[ ]
A. used to get up; taking
B. used to get up; take
C. were used to getting up; take
D. were used to get up; take
