单项选择题

Billy was four years old and he was a very clever boy. He liked playing in the open air very much. Every day after lunch his mother told him to stay at home. She took him to his bedroom and put him on his bed to rest for an hour, but Billy never slept and usually he made a lot of noise and got off his bed every few minutes.
One afternoon, Billy’s mother put him on his bed and then went to her bedroom to do some reading. After ten minutes, she heard a noise, so she went to Billy’s room. He was not there, but his trousers were lying on his bed.
She looked in the other rooms upstairs, but he was not there either, so she went to the balcony(阳台)and shunted down angrily, "Are you running about down there without your trousers"
"No, my dear, "answered her husband. "I have bought some vegetables and I am wearing my trousers. "
Now, Billy was playing in the garden without his trousers.
One afternoon Billy went out without his trousers.