A Decrease in Firm 1's Marginal Cost Will Cause

A decrease in firm 1's marginal cost will cause:

A) a downward shift in firm 1's reaction function, resulting in a new Cournot equilibrium where firm 1 is producing a lower quantity and firm 2 is producing a higher quantity.
B) an upward shift in firm 1's reaction function, resulting in a new Cournot equilibrium where firm 1 is producing a higher quantity and firm 2 is producing a lower quantity.
C) a downward shift in firm 2's reaction function, resulting in a new Cournot equilibrium where firm 1 is producing a higher quantity and firm 2 is producing a lower quantity.
D) an upward shift in firm 2's reaction function, resulting in a new Cournot equilibrium where firm 1 is producing a lower quantity and firm 2 is producing a higher quantity.