How to change span of QWidget inside a QSplitter

Question

I have three widgets disposed as this:

where there is a QSplitter beetween A+B and C

I'm looking for a way to make that, when a certain QPushButton is pressed, Widget C changes its span and lets more space to Widget B, as in this photo:

I tried to change the span of C inside the splitter, but I didn't found any easy way to do that (as there is with layouts). I tried some things but nothing worked as I want.