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.

explanation:

the universal gravitation equation. since the gravitational force is directly proportional to the mass of both interacting objects, more massive objects will attract each other with a greater gravitational force. if the mass of one of the objects is doubled, then the force of gravity between them is doubled.

both male and female gametes are produced by meiosis of the germ cells. therefore, both male and female gametes are haploid. one male and one female gamete unite during the fertilization to form the diploid zygote. in seed bearing plants, pollen grain is the male gametophyte, which produces sperm cells.