The American professor came to realize that he had underestimated the ______ of most of the Chinese students.

【正确答案】 C

【答案解析】 magnitude“大小，重要的程度”；gradient“坡度，倾斜度”；potential“潜力，潜能”；firmness“稳固，坚实”。根据本题句意，underestimated只能与potential搭配，故答案为C。