Answer:
A) when a catalyst is added
Explanation:
The missing options are:
A) when a catalyst is added
B) when the temperature drops
C) when the pressure is lowered
D) when the molecules are large and bulky
A catalyst increases the rate of a chemical reaction without undergoing a permanent chemical change. They provide an alternative pathway to the reaction which has a lower activation energy than the non-catalyzed mechanism, this increase the reaction rate.
