Question on Cost-reimburssable contracts

A project manager must have some work done by an outside contractor. This work has a great deal of risk associated with it, and it has become very difficult to find a contractor willing to take on the job. Which of the following types of contract would offer the greatest incentive to the contractor?




  1. Cost plus percentage of cost as an award fee



  2. Cost plus fixed fee



  3. Cost plus incentive fee



  4. Firm fixed price


Answer: A


Why the correct answer is A, not C. What's the difference between A and C?


Thanks.

 in pmbok 

award and incentive contract are described, based on subjective and objective aspect respectively.

in this question you may undrstand that

award at overall contract work, in broader prospect.

where as incentive is for part subject (objective).

definition / difference of subjective and objective can be referred from wiki