Read the following paragraphs and answer the question.
     Well, all she had was a flat tire, but for an old lady, that was bad enough. Bryan crawled under the car looking for a place to put the jack, skinning his knuckles (指关节）a time or two. Soon he was able to change the tire. But he had to get dirty and his hands hurt. As he was finishing, she rolled down the window and began to talk to him. She said she couldn't thank him enough for coming to her aid.

Which of the following is closest in meaning to “skinning” ？（）