Ali is a student at the University. He recently purchased a car for OMR5,000 to use it for going to the University. Ali also expects that other friends might ask for transportation from him. He expects a total monthly revenue of OMR60. He expects fuel cost to be OMR30 per month. One of Alis friends is a taxi driver. He offered Ali to take him to University for a monthly fee of OMR10. Because he does not have to drive, Ali believes that he can perform online work that would earn him a monthly revenue of OMR10. What is the differential cost in this scenario? Select one:

a. OMR10

b. OMR20

c. OMR40

d. OMR30

e. OMR50