单句语法填空
1．(2014·北京高考改编)Observe (observe) carefully if any change occurs when doing experiments in the lab.
2．(2013·重庆高考改编)It was with the help of the local guide that the mountain climber was rescued.
3．(2013·江苏高考改编)“Never for a second，” the boy says, “did I doubt that my father would come to my rescue.”
