What is the formula for calculating the weighted average cost of capital $($WACC$)?$

a$.$

$($Cost of Equity $*$ Equity Weight$)+($Cost of Debt $*$ Debt Weight$)$

b$.$

$($Cost of Equity $/$ Equity Weight$)+($Cost of Debt $/$ Debt Weight$)$

c$.$

$($Cost of Equity $+$ Equity Weight$)+($Cost of Debt $+$ Debt Weight$)$

d$.$

$($Cost of Equity $-$ Equity Weight$)+($Cost of Debt $-$ Debt Weight$)$