Phil Offers to Do Don's Housework for $20 Per Week

Phil offers to do Don's housework for $20 per week. Don's opportunity cost of doing housework is $30 per week, and Phil's opportunity cost of doing housework is $10 per week.
-According to the information provided, what will be Phil's gain in producer surplus as a result of the proposed transaction?

A) Phil will gain $30 per week
B) Phil will gain $20 per week
C) Phil will gain $10 per week
D) Phil will gain no producer surplus