During a​ year, a​ firm's gross investment is ​$2 comma 000 and net investment is ​$1 comma 600. What is the​ firm's depreciation​? The​ firm's depreciation is​ ______. A. ​$1 comma 200 B. ​$1 comma 600 C. ​$2 comma 000 D. ​$400