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.
Answer: D