The unloading system on an air compressor will （）. ["increase compressor discharge pressure on demand","increase compressor operating speed as necessary","allow the motor to turn the compressor opposed only by friction","reduce the compressor frictional load when starting"]
请设计一个证明植物具有呼吸作用的实验。
称呼指的是什么？
定额中的砂为符合要求的过筛净砂，定额中过筛净砂的消耗量，不包括（）。 ["砂的膨胀系数","过筛损耗","配制各种砂浆、砼时的操作损耗","施工现场的筛砂用工"]
在Excel中，函数SUM（3，5，7，4，2）的值是（）。 ["5","7","21","#VALUE！"]
什么是系统安全？什么是安全系统工程？
