单选题     When the weather is hot, you go to a lake or an ocean. When you are near a lake or an ocean, you feel cool .Why? The sun makes the earth hot, but it cannot make the water very hot. Although the air over the earth becomes hot, the air over the water stays cool. The hot air over the earth rises. Then the cool air over the water moves in and takes the place of the hot air. When you are near a lake or an ocean, you feel the cool air when it moves in. You feel the wind. And the wind makes you cool.
    Of course, scientists cannot answer all of your questions. If we ask, 'why is the ocean full of salt?' Scientists will say that the salt comes from rocks. When a rock gets very hot or very cold, it cracks. Rain falls into the cracks. The rain then carries the salt into the earth and then into the rivers. The rivers carry the salt into the ocean. But then we ask, 'what happens to the salt in the ocean? The ocean does not get more salty every year.' Scientists are not sure about the answer to this question.
    We know a lot about our world. But there are still many answers that we do not have, and we are curious.

单选题     The main idea of the passage is ______.

单选题     You feel cool when you are near a lake or an ocean because ______.

单选题     Now scientists know ______.

单选题     People are always curious because ______.