$9)$ Compatibility Services acquired an $$80,000$ machine on January $1,20$X$3.$ The machine is estimated to have a useful life of $8$ years, and a residual value of $$4,000.$ For units$-$of$-$production depreciation purposes, the machine is expected to produce $400,000$ units. If Compatibility Services uses units$-$of$-$production depreciation, and the company produces $40,000$ units in $20$X$3$; $60,000$ units in $20\%4,50,000$ units in $20$X$5$; and $30,000$ units in $20$X$6$; what is the net book value of the machine at December $31,20$X$6?$

A$)$ $$45,800$

B$)$ $$49,600$

C$)$ $$48,000$

D$)$ $$42\mathrm{.}000$

E$)$ $$40,000$