—You were not listening；what troubled you?—I         my coming math exam．

 It’s obvious that she ______ him for a long time.

（湖南省怀化市2014年高三第二次模拟考试）24. My mother _____ all the cleaning for my family, but recently she has been too busy teaching to do it. 

Allen has decided that he will be an engineer by the time he ________ twenty.

With the gas _____, they had to quit the car and walked.