如图所示，皮带轮的半径R_A=2R_B，A轮半径上一点c和圆心距离为 $\frac{1}{2}$ R_A，a、b分别为A轮和B轮边缘上的点，当皮带轮工作时，皮带与轮不打滑，则a、b、c三点向心加速度之比a_a：a_b：a_c为（   ）

若集合P＝{1,2,3,4}，Q＝{x|0＜x＜5，x ∈R}，则(　　)

C

    Hi! I'm Alan. This is my dream house (梦幻屋). It has a small but nice room. Some pictures are on the wall. There is (有)a bookcase in my room. There is a desk and a chair in it. My schoolbag is on the desk. Oh! Where is my pencil case? It is under the chair. There are some pencils and a ruler in the pencil case. My shoes and a basketball are under the bed. I like my dream house very much.

近年来，雾霾天气在我国频繁出现，空气质量问题已引起全社会高度关注。下图是气温垂直分布的4种情形。读图完成下列问题。

图中最有利于雾霾大气污染物扩散的情形是（  ）

下列选项属于特异性免疫的是(　　)