What is causing the roots of a plant to grow downward

Which statement correctly describes other ways in which nebulae and stars are different? a. a star always has a higher density than a nebula. b. stars can form inside a nebula but a nebula can never be produced by any star. c. stars can never form inside a nebula but a nebula can be produced by any star. d. a nebula always has a higher density than a star. reset submit

What would most likely happen if green plants were exposed to longer days of sunlight? a. the mitochondria would produce less energy . b. the chloroplasts would produce more energy. c. the cell wall would become thicker. d. the vacuoles would quickly shrink. b. the chloroplasts would produce more energy.

Ihave no idea how to solve these. plus there's another question that is 2) a 10 kilogram meteorite is collected in space and brought back to earth. it is composed of 1.25 kg of uranium-238 and 8.75 kg of lead-206. how many u-238 half lives have passed? what is the approximate age of the meteorite?