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答案：B
解析：
Choice B is correct. Let t represent the time, in seconds, that Sam spends running, let d represent the distance run, and let r represent the rate at which Sam is moving.
Then, since d = rt, 5t represents the total distance, in meters, Sam runs in t seconds.
Since Lisa gives her little brother a 15-second head start, it follows that she runs for t-15 seconds at a rate of 8m/sec.
So, 8(t-15) represents the total distance, in meters, Lisa runs in t-15 seconds.
