翳如白云，仔细检查才可看见者为：
A．冰瑕翳B．云翳C．厚翳D．斑脂翳E．凝脂翳
参考答案：B
(2013·湖南省部分重点中学5月联考)—What are they up to？It’s so noisy.
—Some students________a party.
A．have had
B．are having
C．had had
D．were having
试述阀门定位器在调节阀控制中起何作用？
