CO2气体保护焊，焊丝伸出长度过长，（）严重，（）变差。合适的伸出长度应为直径的（）倍。
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A portable foam fire extinguisher is placed in operation by（）
A. turning it upside down
B. pressing the foam lever
C. squeezing the grip handle
D. opening the hose valve
2.4GHz频段：由于存在较多干扰，建议采用20M带宽，选择（）三个互不重叠的信道进行规划组网。
制备的优质泥浆指标，泥浆比重为1.03-1.1，黏度为18-22s，含砂率小于（）%，胶体率大于（）%，酸碱度为8-10。
Advection fog is most commonly caused by（）.
A. air being warmed above the dew point
B. saturation of cold air by rain
C. a rapid cooling of the air near the surface of the Earth at night
D. warm moist air being blown over a colder surface
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