a salvage value of \( \$ 7,000 \) after 4 years. At a MARR of \( 12 \% \) per year, when comparing the alternatives, the equation of PW is written as: a. \( P W X=-20,000-9000(\mathrm{P} / \mathrm{A}, 12 \%, 4)+5000\left(\mathrm{P} / \mathrm{F}_{1}, 12 \%, 4\right)-15000(\mathrm{P} / \mathrm{F}, 12 \%, 4) \) b. \( P W X=-20,000-9000(P / A, 12 \%, 4)+5000(P / F, 12 \%, 2)-15000(P / F, 12 \%, 2) \) c. None of these answers d. \( P W X=-20,000+9000(P / A, 12 \%, 4)+5000(P / F, 12 \%, 4)-15000(P / F, 12 \%, 2) \)