Definitely in C Major. As long as the bass is playing that "main riff" of the song, it should sound right. If the bass wants to deviate from the "main riff", then he too should stay in C Major (aka C Ionian).
I love jamming on this song, it can go on forever just dancing around that C Major scale (although some in my band say I go on TOO long, but hey...what can you do?)
Play music, expand the mind, inspire...
"We must embrace pain and burn it as fuel for our journey" - Japanese poet Kenji Miyazawa