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When the radius of a blood vessel increases, the principles of fluid dynamics, specifically Poiseuille's law, come into play. This law indicates that the flow rate of a fluid through a cylindrical vessel is directly proportional to the fourth power of the radius. Therefore, a small increase in the radius of the blood vessel leads to a significant increase in blood flow.

In biological systems, resistance to flow is inversely related to the radius of the vessel. As the radius increases, the resistance against the flow of blood decreases. This means that the larger the vessel, the easier it is for blood to flow through, resulting in increased flow while simultaneously reducing resistance.

The other options suggest scenarios where flow either decreases or remains constant or where resistance remains constant, which is inconsistent with the physiological understanding of blood flow dynamics. An increase in vessel radius leads to a reduction in resistance, which promotes an increase in flow. Thus, the correct response is that resistance decreases and flow increases.