A
Below is a question with multiple-choice answers. Choose the correct option based on your reasoning.

### Question:
A hydrologist studies the location, movement, and quality of water. A hydrologist recognizes the importance that water exists as ___.

### Choices:
A. a solid, a liquid, and a gas (vapor)
B. a solid and a liquid
C. a solid and a gas (vapor)
D. a solid only

### Answer:
