题目

She had just finished her homework        her mother asked her to practice playing the piano yesterday. A. when            B. while           C. after           D. since 答案：A如图是利用排水法收集好的某无色气体．请椐此回答该气体应具有的物理性质（回答两条即可，不能再回答“无色气体”）    ．