The refrigeration system low pressure cutout switch is actuated by （）. ["an electric system manager","a sensing bulb at the tail coil of the evaporator","pressure changes in the suction line","failure of the king solenoid valve"]
污泥浓缩罐（T-D13A/B/C/D）的出口污泥含水（）。 ["92%","50%","70%","80%"]
直接营销的服务对象主要是（）。 ["中小企业","民营企业","大型企业","国有企业"]
井巷施工方法，以凿岩爆破为主。
简述鱼怪病的症状和病理变化。
为什么放大器电路中要用负反馈？
