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How the hell did he crash this project?

The following question (see image below) is from Farndale's 262 free questions. I can't seem to understand how come the answer is C.

According to the explanation given he is crashing activity START and D thus saving $13000. But how the hell did he come up with that? He is asking "what is the least cost to crash the schedule to 10 days" and for that he is adding "Normal Duration" + "Crashed Duration" of both START and D activities to come up with 10 days!??

i.imgur.com/lyrJ7.jpg

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