Which best explains why blood and the cytoplasm of cells are mainly composed of liquid water?
a. water is a common substance and is readily available.
b. water is an important transport and reaction medium.
c. water prevents biological reactions from occurring readily.
d. water prevents turgor pressure from making the cells too rigid.

The answer is not D, because water creates turgidity.
The answer is not C, because water is important in many biological reactions.
The answer is either A, or B. Knowing that water composes almost 75% of the body, you can assume that It is a common substance, for A, or you could know that is allows reactions to occur more easily, acting as a sort of catalyst, for answer B. 
A or B. Your call for the final answer.

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