老师您好！He can't help___the happy life when he was a child.
A. to llok back upon B. Look back upon
C. looking back upon D. looking back
老师您好！A Russian plane broke up in the air,_____all 224 people on board which was be lieved_____by a bomb planted by Islamic State.
A. killing; to have been caused B. having killing; to have been caused
C. killing; to be caused D. having killed; to have caused