Beginning with an explanation of gravity's effect on a nebula, explain how we humans owe our existence to gravity.

When nebulae collapses due to their own weight, this is due to gravity. The gravity causes atoms to fuse and a protostar is born from the fusion of hydrogen. Fusion of hydrogen forms heavier elements such as helium and more fusion of helium forms carbon. Carbon is the significant element of life and is formed in the center of stars due to nuclear fusion as a result of gravitational force.

organisms are more likely to be more preserved in mud than sand because mud is moist and wet, while sand is very dry.

the answer is:  

line graphs show patterns in data with variables that change over time; whereas, circle graphs show data as parts out of a whole (such as out of 100%).