Good quiz. I would take issue with two of your answers. The largest river is not the same as the longest river. For length there's not much to choose between the Nile and the Amazon. For size, it's not even close - the Amazon is the correct answer. Similarly, the size of a waterfall is not to be reckoned solely by its height but rather by the product of its height and its average flow. On that measure, I think that Victoria Falls would win hands down. Would you agree?
"Largest" was meant to be an overlapping term. In the first caveat I said that measurements are taken by length, height, or area, whichever was most applicable. So for rivers this would mean the longest, and waterfalls the tallest. Also, according to Wikipedia, which is the source I used,(https://en.wikipedia.org/wiki/List_of_rivers_by_length) the Nile is the longest river by length. Hopefully this clears up some confusion :)