Who to start at rb2/ flex in PPR?

Which two out of of these 5 should I start:
Demarco Murray
Joe Mixon
Emmanuel Sanders
Alfred Morris
Cooper Kupp

Mixon has starting role sure up. But he’s playing very poorly
Murray is an injury risk every time he steps on field
Who knows what’s going on with Morris and cowboys atm but think you have better options.

I like kupp over sanders. And pick your poison from RBS. I’d prob roll it with Mixon

Murray and Morris have the most upside.