When contrasting arteries and veins ,which of the following statements is true?
a. veins have thicker walls than arteries.
b. arteries have a thicker tunica media than veins.
c. veins have a tunica media while arteries do not.
d. arteries have valves, but veins do not.
e. arteries have smooth muscle in their walls, but veins do not.

he movement of air through earth's or any planet's atmosphere is called wind, and the main cause of earth's winds is uneven heating by the sun. this uneven heating causes changes of atmospheric pressure, and winds blow from regions with high pressure to those with low pressure.

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brain development. a child's brain undergoes an amazing period of development from birth to three—producing more than a million neural connections each second. the development of the brain is influenced by many factors, including a child's relationships, experiences and environment.

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