I know the right answer but I need help understanding WHY it's the right one.

Quality issues have plagued your project and caused cost to grow and schedule to be delayed multiple times throughout the first four months of the twelve months of schedule. During a recent project update meeting, you communicated that the team has completed $$250,000$ in value and spent $$325,000$ to$-$date. Your sponsor is aware that the team is currently behind schedule and cost and will likely overspend the $$900,000$ project budget. She has asked you to calculate the team's efficiency in completing the project work. What metric and value would you provide to the sponsor?

A SV$=-$$$40,000$

B SV $=$ $$40,000$

C SPI $=0\mathrm{.}78$

D SPI $=0\mathrm{.}85-$ this is the right answer but why?