Answer:
A. temperature of the reaction
B. amount of enzyme
E. pH of the reaction
Explanation:
Temperature should be kept constant because enzymes operate at different activity rates depending on the temperature. The same goes for the pH as well with different enzymes having different optimal pH levels.
The amount of enzyme should be kept constant as well because different concentrations of enzymes can determine how fast the reaction will occur.
Essentially anything that could affect enzyme activity apart from substrate concentration should be kept constant.