答案：A

解析：

Choice A is correct. The time it took Erica to run one mile decreased by about 15 seconds for every 2 weeks of the season. This is equivalent to a decrease of 7.5 seconds every week.

If Erika trains for x weeks, 7.5x represents the total time, in seconds, that she has shaved off the time it took her to run one mile.

Subtracting this from her starting time gives the approximate time (in seconds) it takes her to run one mile at week x.

500-7.5x

Since it takes her 7 minutes and 5 seconds, or 425 seconds to run a mile after x weeks of the season, we can create the following equation:

500-7.5x=425

The equation that best represents the context is 500-7.5x=425.