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 change
B．A supplier went out of business and a new one needed to be foun
C．Additional equipment needed to be purchase
D．A critical path activity took longer and needed more labor hours to complet
