Scenario 8-2 Lincoln Offers to Do Katelyn's Housework for $80 Per Week.Katelyn's

Scenario 8-2
Lincoln offers to do Katelyn's housework for $80 per week.Katelyn's opportunity cost of doing housework is $100 per week, and Lincoln's opportunity cost of doing housework is $60 per week.
-Refer to Scenario 8-2.What is the total gain in welfare due to the transaction described above

A) $10 per week
B) $20 per week
C) $30 per week
D) $40 per week