Detailed Instructions: You are given a sentence and a question in the input. The information provided in the sentence should be enough to answer the question. You're expected to write the correct answer. Do not use any facts other than those provided in the sentence.
Q: Sentence: One day Mark was out playing football when he tripped and hurt his knee, the next day Matthew was also playing football when he fell and hurt his arm. 
Question: Which of the boys hurt his knee, and from what?
A:
Mark hurt his knee from tripping.