Confusing...

One good thing about the Internet is __ students can do research for their homework or sometimes practice foreign languages by chatting online with their friends in other countries. Also, it’s a good way to keep in touch with friends and families around the world. But students may get too into it, spending too much time online __ spending less time exercising.
(A) why ; so (B) that ; while (C) why ; so that (D) that ; but

Hi, I'm really confused on option C and D... Which one is the answer?