The Correct answer is A,
This is the only one that makes sense, because there are many different types of rivers, ranging from straight and deep to curvy and shallow.. vice versa. It's not B because you're talking about rivers and they really can't be controversial. It's not C because, It's a research paper and should be difficult. It's not D because rivers are parts physical landscapes of the world which is basically all the class talks about. Please mark Brainiest!!!!