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 8 m/sec.

So, 8(t-15) represents the total distance, in meters, Lisa runs in(t-15) seconds.

If Lisa catches Sam, they would have covered the same distance. Therefore, we can set up the following equation and solve for t:

8(t-15)=5t

8t-120=5t

3t=120

t=40

Since t represent the time, in seconds, that Sam spends running, this shows that Sam will run for 40 seconds before his sister catches him.

Since Lisa runs for t-15 seconds, it follows that she runs for 25 seconds before catching up with Sam.