我国提倡一对夫妇只生一个孩子，从种群特征来看，这种措施直接调节（）
A.种群密度
B.性别比例
C.年龄组成
D.出生率
参考答案：D
During the class, Tom slipped out of the classroom ____the back door so as not to be found by the
teacher.[ ]
A. across
B. in
C. by
D. beside
呼吸性粉尘是指直径多大的粉尘（）。
A.＞15μm
B.＞10μm
C.＝10μm
D.＜8μm
E.＜5μm
