Which Flex Should I Start??

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I'm already startin Charles vs Cle and Maroney @ Buf for RB and Fitz @ Det and S. Rice @ Car for WR. Which Flex option will get me the most points to go along with that?

Ganther vs NYG
A. Foster @ STL
Grant @ Pit
D. Williams vs Min
Britt vs MIA

My team:

QB: Romo
RB: Charles
RB: Maroney
WR: Fitz
WR: S. Rice
WR/RB: ?
TE: F. Davis
D/ST: Ravens
K: Stover
BN: D. Williams, R. Grant, J. Stewart, Buckhalter, A. Foster, Q. Ganther, Britt, Sims-Walker, Wallace

10 team H2H Standard Scoring non PPR