题目

      Shanghai Students’ Post ______ a great help in my English learning since three years ago.       A) will be                   B) was                      C) has been            D) had been 答案：C现有O2和NO2的混合气体12 mL，被水充分吸收后，剩余气体2 mL，则原混合气体的组成是 [　　] A．NO211 mLO21 mL B．NO210.8 mLO21.2 mL C．NO210 mLO22 mL D．NO28 mLO24 mL