Please write out correct answer and whether it's A, B, C or D. Thank you.

Calgary Lumber Company incurs a cost of $40 per hundred board feet in processing certain "rough-cut" lumber, which it sells for $51 per hundred board feet. An alternative is to produce a "finished-cut" at a total processing cost of $55 per hundred board feet, which can be sold for $70 per hundred board feet. If Calgary chooses to produce "finished-cut", how much is differential income? O $ 4 per hundred board feet O $17 per hundred board feet $ 21 per hundred board feet O $ 11 per hundred board feet