5points
why do comets have tails?
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a. the sun's gravity pulls them apart.
b. comets are unstable and naturally disintegrate.
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c. earth's gravity pulls the tail toward earth.
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d. the sun vaporizes part of the ice of the comet.
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Consider the model of the oxygen cycle on earth. the majority of earth's oxygen reservoirs are found in the a) atmosphere. b) biosphere. c) hydrosphere. d) lithosphere.

Variety of living organisms; includes genetic, species, and ecological types.

What sentence best supports the statement that hormones are involved in the regulation of homeostasis? a. the hormone erythropoeitin increases the production of red blood cells when oxygen levels are low. b. the hormone oxytocin promotes labor contractions of the uterus during childbirth. c. the hormone melatonin induces sleep and its production is slowed by exposure to light. d. the hormone cortisol suppresses the immune system and is produced when the body is under stress.