Which sentence is a run-on and should be written as two sentences?

A. Watching any movie is like entering a world of make-believe a movie with special effects makes it an even more magical illusion.

B. I have wanted a pet guinea pig since I was a little kid, but my brother is allergic to animal fur.

C. Though Jamie was an accomplished piano player, she decided she would like to learn to play the saxophone, as well.

D. While whale watching is a fun activity for most people, it is less enjoyable for people who get seasick.