Which phrase best defines a cell?
a. the most basic structural unit
b. the physical attributes of a unit
c. the structure of several tissues
d. the structure of a plant or animal

The answer is A.

Cells are the most basic structure to all life. In order to have tissues you need two or more cells. If you have two or more tissues you make an organ. If there are two or more organs you have an organ system. If you have an organ system then you have a living organisms such as a human!

Hope that helped :) I'm taking human anatomy in college.

The role of carbon dioxide is to provide the matter for sugar which is the main output of photosynthesis.