i dont understand double declining method . i thought the answer was 30000

Smith Farms purchases a combine on January 1, 2015 for $50,000. Using the double declining balance method for depreciation, a 5 year lifetime and a $5,000 salvage value, what would be the book value of the combine December, 31, 2016? A) $18,000 B) $20,000 C) $22,000 D) $30,000