Read the following context and choose the best option to answer the question.
Context: Can I say , Yesterday was a rainy day . John fell down and hurt his knee . ( a ) John tried to walk but failed . Luckily , he saw two scouts was on the way to him .
Question: Why may have John hurt his knee ?
Options: 
- The scouts pushed him .
 - He slipped on a wet surface .
 - He was pushed by the scouts .
 - He was fooling around .


He slipped on a wet surface .