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