Mitcham Ltd has observed that at an activity level of 10 000 units the maintenance cost is $12 000, and at 20 000 units the maintenance cost is $18 000. Using the high-low method, the cost formula for maintenance is:

Select one:

a.$5000 + $.60 x units.

b.$1.54/unit

c.$6000 + $.60 x units.

d.$6000 + $.1.54 x units.