Hi guys, quick Procurement question I need help on.

Question: Of the following contract types, which exposes the seller to the highest risk?

A. T&M

B. FFP

C. FP-EPA

D. CPAF

I chose FFP, but the answer apparently is D (CPAF).  CPAF is Cost plus award fee.  How can this be the riskiest to the seller when all costs are being reimbursed by the buyer?  (they may not get a fee if they don't perform, but their costs also won't explode?)