White Co$.$ purchased equipment on July $6,20$X$7$ for $$96,000.$ The equipment has an estimated salvage value of $$4,000.$ The estimated output that the equipment is expected to provide is $25,000$ units. White Co$.$ uses the units of output depreciation method. During $20$X$7,2,500$ units were produced. During $20$X$8,3,200$ units were produced. What was the book value of the equipment on December $31,20$X$8?$

a$.$ $$96,000$

b$.$ $$75,024$

c$.$ $$20,976$

d$.$ $$74,112$