What the number is the main idea? i need answer.

The main idea is supposed to introduce what would be talked about later in a more broad term. In this case, sentence 1 should be your answer, as it not only introduces the problem, but also what the solution should aim for (in this case, the elimination of trash). All sentences afterwards gives examples on how society had deposed of waste as history progresses, and can be seen as the body part of the paragraph (or the answer).