What organism is the main producer in freshwater ecosystems? a) phytoplankton b) zooplankton c) anaerobic bacteria d) freshwater weeds e) none of the above

Mars has a surface gravity that is only 0.38 (a little over one-third) that of earth's. thus, the weight of an object will also be 0.38 times that of earth. what is the g.p.e. of a climber who weighs 675 newtons on earth at the top of 26,000-meter olympus mons?

Which of these is a disadvantage of hydrogen fuel cells? a. engines that use hydrogen fuel are more efficient than gasoline-powered engines. b. the only emission produced by hydrogen fuel cells is water. c. hydrogen fuel is continually available and useful for heating. d. hydrogen fuel is often purified in a process that emits lots of greenhouse gases.

What happens when two galaxies approach each other? a. they form a black hole. b. they merge because of the strong gravitational pull they exert on each other. c. they shred each other into smaller galaxies. d. they form a supernova. e. they repel each other away because of their strong gravitational pull.