This contract type shall not be used unless—

(a) The contract is for research and development and the estimated cost is the simplified acquisition threshold or less;

(b) The contractor's accounting system is adequate for price redetermination;

(c) There is reasonable assurance that the price redetermination will take place promptly at the specified time; and

(d) The head of the contracting activity (or a higher-level official, if required by agency procedures) approves its use in writing.