Which of the fluids below would produce the highest pressure at the bottom of a tank for the same fluid depth? a. mercury b. air c. water d. helium?

The answer is  a property of density. The higher the density, the higher the pressure at the bottom. 

Pressure = mass / Area. So given that the 4 samples occupy the same area at the bottom, the mass is going to be the determining factor. Per given volume, mercury has the largest mass. The answer is A

One splits the atoms and molecules  the other combines them

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