根据短文内容，选择正确答案。

    David did well in all his subjects except physics, Mr. Brown, his physics teacher, gave him his physics paper and said to him, “Show it to your dad and ask him to sign his name on the paper.”

    David played football until it was 7 o'clock. When he got home, his parents had finished supper. His dad was watching TV and his mother was doing the dishes in the kitchen as usual.

 “Dad, may I ask you some questions?” David asked happily.

 “Yes, of course,” answered his dad.

 “If you can't answer them, you'll have to do something for me.”

 “OK, I'll do as you tell me,” said his dad.

 “Which is cooler, walking or sitting in the sun?”

 “Walking is cooler. It can make wind.”

 “Which clothes can keep out (阻挡）the sunlight，shirts or coats?”

 “Coats can keep out the sunlight because they are much thicker than shirts.” “Is a man smart if he wears a coat and walks in the sun in summer?” David looked at his dad and waited for his answer. His dad thought for a minute and shook his head.

   David took out his paper, “Now sign your name on the paper, please.”