可吸入颗粒物的直径大小是（）
A.<100μm
B.≤10μm
C.≥5μm
D.1～5μm
E.0.1～5μm
参考答案：B
About six months later，the letter they were looking forward to ________at last．
A．arrived
B．arriving
C．had arrived
D．arrive
阀门使用维护的一般方法有哪些？
