担保方式中的保证，实际运用过程中应理解为( )。

A．债务人和债权人约定，债务人向债权人保证履行合同义务

B．债务人和债权人约定，当债务人不履行债务时，由保证人代为履行债务

C．保证人和债权人约定，当债务人不履行债务时，保证人按约定履行债务

D．保证人和债务人约定，当债务人不履行债务时，保证人按约定履行债务

During a project the scope of a product purchased on a cost reimbursable contract has increased．In the contract the fee to cover the contractor's indirect costs and allow a profit is calculated as 20% of the direct costs．What is most likely to be true ？（）

A．The contractor's indirect costs will increase and the customer's payment for them will increase，to

B．The contractor's indirect costs will increase but the eLlstomer's payment for them will not increas

C．Neither the contractor's indirect costs nor the customer's payment for them will increas

D．The contractor's indirect costs will not increase but the customer's payment for them will increas