题目

下列装置图中，中间的试管收集浓硝酸与铜丝反应所生成的气体，何者可使收集到的气体中空气的含量最少（ ） 答案：C解析：浓硝酸与铜产生的气体为NO2，密度比空气大，要使收集到的气体中空气的含量最少，必须用向上排空法，所以进气导管必须伸入试管底部。She ______ the sign language before she served as a volunteer to help the deaf.A．learns B．has learnedC．was learning D．had learned