What I would like to do is quite particular, so I've drawn a picture to illustrate it:
I can easily implement this:
but it doesn't look good on this site because the E element is much taller than C or D.
What I would like, is for C and D to stack when the browser window is wide, but not when it's medium width.
I'm trying to implement this with CSS and Flexbox, and I've tried grouping together C and D in a div, but this creates problems at the medium layout.