节约能源应从我做起，我校教室共有规格为“220V、60W”的日光灯100盏，如果做到人离灯熄，人少时少开灯，那么按每盏灯每天少开1小时计算，1天可节约______kW•h的电能．

A manufacturing project has a schedule performance index(SPI)of 0.89 and a cost performance index(CPI)of 0.91．Generally，what is the BEST explanation for why this occurred ？（）

A．The scope was changed

B．A supplier went out of business and a new one needed to be found

C．Additional equipment needed to be purchased

D．A critical path activity took longer and needed more labor hours to complete