$(a)$ What happens to the heart when muscles work harder?
$(b)$ Which body system is directly affected when a person has heart disease?
$(c)$ Which cells increase in number during infection?

(N/A) When muscles work harder,they require more oxygen and nutrients. Consequently,the heart beats faster to increase the rate of blood circulation to meet this increased demand.
$(b)$ The circulatory system (or cardiovascular system) is directly affected when a person suffers from heart disease.
$(c)$ White blood cells (also known as leukocytes or $WBCs$) increase in number during an infection to help the body fight against pathogens.