The Accompanying Table Describes the Relationship Between the Number of Workers

Multiple Choice

The accompanying table describes the relationship between the number of workers hired by a call center each hour and the number of calls the call center can make each hour. The call center has only 1 telephone. The telephone costs the firm $5/hour (regardless of how many calls are made) , and each worker is paid $10 per hour.  
Given the information in the table above, what is the call center's marginal cost when it goes from making 6 to 16 calls an hour?

A) 50 cents
B) $2
C) $10
D) $20