The beach may seem like an inviting place to run on a cool summer morning, but is running on the sand good for your body? It can be, but it's important to be aware of the challenges that some beach runners experience before you get ready.
Running on the beach can be a good thing. It's certainly different from running on a track or a pavement. For starters, sand can be a challenge because it has an uneven （不平坦的）surface. As you push off, you^?re going to lose some of
 your push as the sand moves. So, you're not going to be able to push yourself forward as you would on a track or a pavement. But this unevenness has an upside： It gives your body an extra workout, forcing you to exercise muscles that don't get as much use during runs on firm surfaces. For example, your feet, ankles and lower back might feel sorer and more tired than usual after a beach run just because the surface is constantly moving and changing step to step.
You also might feel sore afterward because beaches tend to slope （倾斜） down to the water. If you're going for a long run on the beach, you might notice that one side of your body might feel sorer because you're putting more pressure on it due to the slope.
Some beach runners prefer to run barefoot — not wearing anything on feet. However, if you're not used to barefoot running, start slowly and don't run long distances at first. That's because running barefoot uses more different muscles than running with shoes does, and it's important to strengthen these muscles and adapt your feet. If you want to run barefoot and as long as you run carefully enough into it, go for it!
Running on the sand can be a smart choice for you, because running on softer ground surfaces can reduce muscle damage.
16. Why is it difficult to run on the beach for starters?
A. Running on its uneven surface takes much effort.
B. People need to run carefully on its smooth surface.
C. The soft surface makes people fall over easily.
D. It's hard to keep one's balance on its hard surface.
17. Which of the following best explains “upside" underlined in Paragraph 2?
A. Character. B. Reason. C. Advantage. D. Challenge.
18. The passage can probably be found in .
A. A primary school textbook B. A sports magazine
C. A tourist guide D. A scientific report
 16-18 ACB