The Above Table Shows the Probability Distribution of Cake Sales

The above table shows the probability distribution of cake sales at Busy Betty's Bakery.
-Refer to the table above. Busy Betty sells her cakes for $20 each and her constant marginal cost to produce each cake is $12, which is equal to her (constant) average total cost. What is her expected marginal benefit from holding the 21st cake in inventory?

A) $6.10
B) $6.40
C) $8.00
D) $7.20