Doctors suggest that during long periods of sitting, you should flex and extend your feet at the ankles periodically and squeeze your calf muscles to assist with blood circulation.

Why would non-exercise muscle contraction affect blood flow and cardiovascular health?

This helps prevent blood pooling and clotting in the lower extremities, which can lead to deep vein thrombosis and pulmonary embolism.
It also reduces the workload on the heart and improves blood circulation throughout the body.
Choice A is wrong because contraction of muscles does not stimulate the arteries to vasodilate, but rather causes vasoconstriction in the inactive tissues to divert blood flow to the active muscles.
Choice B is wrong because the contraction of muscles itself does not increase heart rate significantly unless it is part of an aerobic exercise that elevates the metabolic demand.
Burning calories and keeping the heart strong are benefits of regular physical activity, not occasional muscle contraction.
Choice C is wrong because contraction of the leg muscles does not stimulate baroreceptors, which are pressure-sensitive receptors located in the walls of the arteries.

Baroreceptors respond to changes in blood pressure and signal the autonomic nervous system to adjust heart rate and vascular resistance accordingly.
Contraction of the leg muscles does not affect blood pressure or heart rate directly but rather assists with venous return.

This is because blood type AB has both A and B antigens on the surface of red blood cells.

Antigens are protein molecules that can trigger an immune response if they are foreign to the body.
Choice A is wrong because antigen B only is present in blood type B.
Choice B is wrong because neither antigens A nor B are present in blood type O.
Choice C is wrong because antigen A only is present in blood type A.
Normal ranges for blood types vary by population, but according to the NHS, about 3% of people in the UK have blood type AB.