这篇文章很难 学生们花费了大量的时间学习它。()

这篇文章很难，学生们花费了大量的时间学习它。()

A.The text is so difficult that the students spend plenty of time learn it.

B.The text is so difficult that the students spend plenty of time to learn it.

C.The text is so difficult that the students spend plenty of time learning it.

请帮忙给出正确答案和分析，谢谢！