Answer:
C. The length of one lap.
Explanation:
In the first four words, the problem statement tells you that L is the number of laps run. It is not the length of one lap.
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Comment on the problem presentation
Appropriate punctuation would be very helpful. Apparently, it is a 1/4-mile track, not a 14-mile track. Apparently, the distance is 1 1/2 miles, not 112 miles. Copying and pasting problem text often leaves out the special symbols used on some web sites. Some editing is usually needed.