Stackelberg Duopolists, Firm 1 and Firm 2, Face Inverse Market $P = 50 - Q$

Stackelberg duopolists, Firm 1 and Firm 2, face inverse market demand $P = 50 - Q$ . Both have marginal cost, $M C = \$ 20$ . If the follower takes the leader's output as fixed at Q₁, what is the equation of its reaction function?

A) $30 - Q _ { 1 } = Q _ { 2 }$
B) $15 - Q _ { 1 } = Q _ { 2 }$
C) $15 - 2 Q _ { 1 } = Q _ { 2 }$
D) $15 - Q _ { 1 } / 2 = Q _ { 2 }$