If Im planning to apply for graduate school in one of the top 20?
It hugely depends. Are you an American or foreign national? Which graduate school, which subject? What subject did you get that C in? What do your GREs look like? Do you have any research experience in the area of the graduate program? Basically, if you are an American straight A student and that C is in a subject unrelated to the topic of the Ph.D. program, no one would care (if could even marginally help your application depending on the admissions committee since it makes you look human). H...