What is the passage mainly about?

Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions from 29 to 33.
If you walk into the streets and see someone throwing a plastic bottle on the ground, you’ll probably get annoyed. But how do we react in the same way when we see people throwing rubbish into the sea? We all know how rubbish ruins the environment on land, but we often forget the influence that it can have on environments like the sea, lakes, and rivers, too.
First of all, water pollution looks terrible. Many beautiful beaches can become covered in rubbish when whatever we have thrown into the water comes ashore. Even rivers and lakes have some plastic bags and bottles floating in them.
Secondly, rubbish can hurt animals and birds that live in or by the water. If they see a plastic bottle, they may think it is food. However, when they try and eat the bottle, it can get caught in their mouth or stomach and stop them from eating anything else. Plastic bottles can also stop dolphins from breathing. Sometimes, fishing boats leave bits of fishing net behind in the water. Fish can get caught in these and die.
Finally, people forget about chemicals that stay in the water. This is very bad for both fish and plants. If you eat fish containing these chemicals, then you can also get ill.
In conclusion, we need to worry about water pollution as much as we care about land or air. We should all protect the seas, lakes and rivers, and remember to take our rubbish away with us.
(Adapted from Empower)