The cash flows for four projects are shown below, along with the cost of capital for these projects.

If these projects are mutually exclusive, which one should be taken?

A$.$ Year: $012345$

Cash flow: $-$$$20,000$$$6000$$$6000$$$6000$$$6000$$$6000$

Cost of Capital: $8\mathrm{.}6\%$

B$.$ Year: $012345$

Cash flow: $-$$$15,000$$$4000$$$4000$$$4000$$$4000$$$4000$

Cost of Capital: $7\%$

C$.$ Year: $012345$

Cash flow: $-$$$18,000$$$5000$$$5000$$$5000$$$5000$$$5000$

Cost of Capital: $8\%$

D$.$ Year: $012345$

Cash flow: $-$$$12,000$$$4000$$$4000$$$4000$$$4000$$$4000$

Cost of Capital: $5\%$