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The main idea of this passage is that
A.
sharks can attack any time, so people should stay out of the water entirely.
B.
sharks attack many people, but they make their attacks in different ways.
C.
there are many reasons that explain why sharks attack people.
D.
sharks are just curious, so people should not panic if a shark bumps them.

Introduction
Curiosity
Increase in Water Recreation
Hunger
Changes in the Environment
Conclusion
Terrifying, isn't it? Just the very thought that you could be kicking and splashing in the waves, the salty ocean air blowing through your hair, the sand pushing up between your toes, and—WHAM!—something hits you. But it's not just a friend bumping into you. It's a shark. You've just been attacked in water no deeper than your waist as you swim and play with scores of other people who are swimming just like you. Scientists know a lot more about sharks than they used to, and they believe that the image of the mindless killer isn't right. So, why did that shark attack you? There are several reasons why sharks attack people and why those attacks are increasing, but those reasons may not be what you think.

Curiosity
By nature, sharks are extremely curious creatures. If there is a foreign object or presence in their territory, they naturally want to check it out. Without hands or feet, the only way sharks can do this is to mouth the objects they encounter. Unfortunately, human beings have thin skin with a lot of small blood vessels close to the surface, so this type of interaction can cause severe damage.

Increase in Water Recreation
Swimming did not become a favorite pastime until the 1930s, and the amount of time that people spend in the water as a means of recreation has steadily increased ever since. It only makes sense that an increase in contact between sharks and humans means the frequency of attacks will also increase.

Hunger
Sharks, like any other animal, have dietary needs that must be met if they are going to survive. An overall decrease in their normal food supply, due in part to an increase in fishing hauls, leaves many sharks hungry all the time. Hungry sharks will go for any food source that seems reasonable, and sometimes that includes humans who have wandered into their territory.

Changes in the Environment
Global warming is taking its toll on humans and animals alike. Sharks are no exception. Waters that were once cold—and therefore preferred by many sharks—have gotten significantly warmer. This change has forced sharks to look for food closer to shore, and that's the best place to run into people. Additionally, agricultural chemicals have drained from the soil and trickled down into the oceans. This chemical change has created "dead zones" in places like the Gulf of Mexico. Dead zones kill aquatic life, leaving sharks with a diminished food supply.

Conclusion
So what does all this mean for people who love to swim in the ocean? It means that we will all need to educate ourselves more about sharks and their habits if we are going to swim safely. It means that we are going to have to be more conscious and considerate of the shark's territory. Most importantly, it means that we have another reason to understand how our actions are impacting the environment and all the other creatures in the world.