During an exothermic chemical reaction,
A. a system becomes warmer, and the chemical substances undergo a decrease in potential energy.
B. a system becomes warmer, and the chemical substances undergo an increase in potential energy.
C. a system becomes cooler, and the chemical substances undergo a decrease in potential energy.
D. a system becomes cooler, and the chemical substances undergo an increase in potential energy.
E. a system becomes warmer, and additional heat is gained from the surroundings.