The earned value technique is a commonly used method of performance measurement. It integrates project scope, cost, and schedule measures to help the project management team assess project performance. If a project's Schedule Performance Index(SPI=EV/PV) value less than 1.0, it means that（）.
A.the project is ahead of its planned schedule
B.less work was completed than was planned
C.the cost overrun for work completed
D.the cost under-run of performance to date
