Under the high-low method, the federal per diem amount is 

a. The same in every city in the U.S. 

b. Different for every city in the U.S. 

c. The same for most cities but higher in certain locations.

d. An average of the highest and lowest costs for that city.

e. The boundary for expenses incurred in a city (never higher than the high amount but never lower than the low amount)