人民防空教育的对象是哪些（）。
A.全体中学生
B.全体民众
C.全体社区居民
参考答案：B
俄罗斯平原与高原地形的分界河是[ ]
A．伏尔加河
B．乌拉尔河
C．叶尼塞河
D．勒拿河
Jack wasn't saying anything but the teacher smiled at him ____ he had done something very clever.
A as if B in case C while D though
