现有①氮气 ②氧气 ③稀有气体 ④二氧化碳四种物质,请按下列要求将序号填入空格内: (1)占空气总体积0.03%的气体是________; (2)可导致温室效应的气体是_________; (3)可制多种电光源的气体是_________; (4)可制氮肥的气体是_________; 答案:④ ④ ③ ①These two maths problems can’t be used in the exam. They are too_________to the students. Everyone can remember the answers. A. easy B. difficult? C. familiar D. simple