Jon, a U$.$S$.$ petroleum engineer, moved his family to Saudi Arabia for a two$-$year assignment. The company provided him a travel allotment of $$\mathrm{.}10$ a mile, a food allowance of $$500$ a month, and $$2000$ a month for housing to maintain his standard of living. What kind of pay factor was used?

a$)$ Base pay.

b$)$ Differentials.

c$)$ Bonuses.

d$)$ Incentives.

e$)$ Assistance programs.