一个地区人口数量的自然增长，取决于[ ]
A．人口的迁移数量
B．人口的总数
C．人口的年龄结构
D．人口的出生率和死亡率
答案：D
—May we leave the classroom now?
—No, you _______. You ________ to leave until the bell rings. [ ]
A. mustn't; are allowed
B. don't have to; are supposed
C. needn't; aren't allowed
D. can't; aren't supposed
关于根管口的解释，正确的是（）
A.根管末端的开口处
B.髓室和根管交界的部分
C.髓腔中分叉的部位
D.髓腔的开口
E.侧支根管的开口
