题目

  Dear Michael,     What a wonderful day it is today! The sun was always shining，but it was cool．I had a great field trip to East Mountain with Grandpa and Danny．The mountain was a little high，so it took US over art hour to reach the top．     Standing on the top of the mountain, I breathed in the fresh air and looked up．The sky was blue and I could see some white clouds fly over my head quickly．I didn't feel very tired，but Danny kept saying that he couldn't move any move. Grandpa to1d us“no pains，no gains”．That is a very good saying and I'11 try to keep it5．A lot of flights were _______ because of the smog（雾霾）last month．（　　）A．put downB．put upC．put offD．put out