A delivery van that cost $40,000 has an expected service life of 180,000 miles and a residual value of $4000 .The van was driven 24,000 miles in the first year and 36,000 miles in the second year .Accumulated depreciation by the end of the second year of the asset's life using the units-of-production method is:

a) 4800

b) 12000

c)7200

d)all the choices are incorrect