contracts MOST risk for the Buyer

Which of the following contracts has the MOST risk for the Buyer

 
A.    Cost plus fixed fee (CPFF) 
B.    Time and Materials (T&M)
C.    Cost plus award fee (CPAF) 
D.    Fixed price (FP) 
 
As per head first it is T&M. But  I choose CPFF. Rita and PMBOK also says the same
 
Can anyone explain this?
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