Which of the following substances is a fluid?
air
ice
sand
candles

Air

Explanation:

Air like all gasses, is a liquid. Liquid is a substance that flows freely. Therefore, your answer would be gas.

(1) Though Ice, can melt to become a liquid, its form is still TECHNICALLY a solid.

(2) Sand can form to shape, but it still considered a solid. Sand is a solid because each grain of sand is just a very small solid that can hold its shape

(3) Candles, are a solid even though, like ice, can become a liquid but only for a short amount of time because wax would eventually turn back into a solid.

- decrease the number of plants and animals living in the water.