Static stretches are part of an effective cool down because they can
a. improve performance
b. decrease blood flow
c. prevent overload
d. increase range of motion

The answer is letter D. 

Static stretches are part of an effective cool down because it can help increase range of motion in joints. It is advisable to perform static stretches after a workout because it reduces the chances of getting muscle pains, it increases the blood flow to surrounding joints of the body, and it removes lactic acid build-up within the muscles. Never stretch a cold muscle before a workout because this may lead to injuries. It is always an important part of a training regimen and should not be taken lightly by individuals as to prevent injuries and improve performance.