Question

q: which blood vessels have valves to prevent blood from flowing backward?
description: sys2 blood v obj1b
a: arteries
b: veins
c: capillaries
d: venules

Explanation:

Veins are blood vessels that carry deoxygenated blood (except pulmonary and umbilical veins) back to the heart. The pressure in veins is lower compared to arteries. To prevent the backflow of blood (especially in limbs against gravity), veins have valves. Arteries carry blood away from the heart at high pressure and do not need valves for backflow prevention. Capillaries are the smallest blood vessels for exchange of substances and have no valves. Venules are small veins that merge to form larger veins, but the key vessels with valves for backflow prevention are veins as a whole category.

Answer:

B. Veins