People Living Under Chronic Stressful Conditions Tend to Get Sick

People living under chronic stressful conditions tend to get sick more often than they would otherwise.How do researchers explain this phenomenon?

A) Chronic stress tends to reduce immune system functioning,thus making us more vulnerable to diseases.
B) Chronic stress leads to a lowering of the heart rate,which makes the heart inefficient.
C) Acute stress makes muscles stronger,which places a greater burden on the heart and respiratory system.
D) The body tends to adapt to the constant call for the stress response and,thus,future responses are not as strong as before.