Read this sentence:

Mr. Knowles figured out that his students had a very hard time understanding quadratic equations.

What is the best way to revise this sentence to be more specific without changing the meaning?

A. Mr. Knowles' students worked hard to comprehend quadratic equations.
B. Mr. Knowles wondered why his students had such a hard time understanding quadratic equations.
C. Mr. Knowles realized his students were struggling to understand quadratic equations.
D. Mr. Knowles' students realized that they did not fully understand quadratic equations.