About 3% of the water on earth is freshwater. only about 40% of that freshwater is available for human use. why is so much freshwater unavailable for human use? a) it is frozen. b) it is polluted. c) it is salt water. d) it is in aquifers. 2) the deepest place in the seafloor is called the a) abyssal plain. b) marianas trench. c) aleutian trench. d) hydrothermal vent. 3) tides are caused by a) gravity. b) heat. c) waves. d) winds.