Answer:
1. B. $8,000
2. C. $7,200
Explanation:
Units or production (UOP) method of depreciation bases the depreciation expense of a machine or equipment on how much it is actually used during the period. 
depreciable value = $50,000 - $10,000 = $40,000
depreciation rate per unit = $40,000 / 25,000 = $1.60
Year          Projected Production units         Actual Production units 
1                              4,500                                    5,000
2                             5,000                                    4,000 
3                             3,500                                    3,000
4                             5,500                                    5,000 
5                             6,500                                    Not known
Total                      25,000
depreciation expense year 4 = $1.60 x 5,000 = $8,000
accumulated depreciation year 4 = $1.60 x 17,000 = $27,200
book value = $50,000 - $27,200 = $22,800
if sold at $30,000, gain resulting from sale = $30,000 - $22,800 = $7,200