In an arrangement of piano variations for chamber ensemble, a cello is tacet in one of them, and the next starts with cue notes from violin, using

Should a cautionary clef change be placed
a) at the end of the previous variation?
b) right after the key signature of the variation in which the cello plays (then showing like



c) at the beginning of the variation, but no cautionary clef change in the previous?
Didn't find info on the web, nor in Gould's Behind Bars...
Thanks,
Felipe Copaja