The novels written by the author sell best, but five years ago no one could have imagined how great a role he _____in the literary (文学的) world.

A．was playing

B．was to play

C．had played

D．played

在容积为10L的密闭容器中，通入40mol N₂和120mol H₂，在一定条件下进行反应，2s后达到平衡状态，此时N₂的浓度为2mol•L^-1．试求：

（1）用H₂浓度表示2s内的平均反应速率

（2）N₂的转化率

（3）平衡时NH₃的体积分数．