Answer:
Differential cost is $0
Step-by-step explanation:
A company should process further a product if the additional revenue from the split-off point is greater than than the further processing cost. 
Additional sales revenue = Sales revenue after further processing - sales revenue after split-off point
. A company should process further a product if the additional revenue from the split-off point is greater than than the further processing cost. 
Also note that all cost incurred up to the split-off point are irrelevant to the decision to process further . 
 $
Sales after split off point (Product C) 58 
Sales at the split off point (Product B) 33
 Additional sales revenue 25
Further processing cost  (25)
Differential cost 0
Differential cost is $0