合同或协议明确规定销售商品需要延期收取价款，如分期收款销售商品，实质上具有融资性质的，应当按照应收的合同或协议价款的现值确定其公允价值，即按应收的合同或协议价款的现值确定商品销售收入金额。( )

In your software development project you have planned for system testing from March 1st and the duration is 10 days．Two days before the completion of the system test，you can start the user acceptance test．The 2 days in this case is an example of （）．

A．lead

B．lag

C．delay

D．float