Explain why people who eat only plants must eat a variety of plants in order to obtain all the nutrients they need

Because different plants contain different vitamins and minerals as well as different amino acids and our body needs a large variety of these in order to preform all the necessary chemical reactions taking place inside our body. Some vegetables are very rich in a certain vitamin but are lacking in another, like how oranges have plenty of vitamin C but not iron. Vitamins and minerals are used in your cells to speed up and allow pretty much any chemical reaction to occur, but each one helps with a certain process and cannot simply be replaced by a different vitamin. In addition, very few vegetables contain all of the essential amino acids that work as the building blocks for protein synthesis and the creation of enzymes, so if you are not getting all of these in your diet, it is very unlikely you are producing enough enzymes and as a result certain functions in your body could stop working altogether.

Not what goes around on earth but the universe too.