The purchase price of an equipment is $45,000 which is expected to drop to $12,000 after 10 years of use. If the applicable depreciation method is the straight line depreciation, what is the book value of this equipment at the beginning of the 7th year? O a. $3,300 O b. $23,100 O c. $21,900 O d. $19,800 O e. $25,200