In one or two sentences explain why people will do less of an activity when the mariginal costs are greater than the mariginal benefits

Going on the assumption that it's about costs greater than benefits or something similar: People will do less of an activity when the costs outweigh the benefits (or the cons outweigh the pros) because it simply isn't worth doing; it causes them more harm than good

parts of an essay — traditionally, it has been taught that a formal essay consists of three parts: the introductory paragraph or introduction, the body paragraphs, and the concluding paragraph. an essay does not need to be this simple, but it is a good starting point.

dramatic irony is the answer