Assume that a bus costs Rs $1600,000$ and is expected to realize Rs $100,000$ at the end of its estimated useful life of four years. If the firm is using straight line method of depreciation, the depreciation expense in the $4$ th year will be:

a$.$ Rs $400,000$

b$.$ Rs $375,000$

c$.$ Rs $500,000$

d$.Rs750,000$