Plants are a source of
food
fuel
medicine
all of the above

Plants are a source of food, fuel and medicine.

Explanation:

In biology, plants are photosynthetic living beings without locomotive capacity whose cell walls are mainly composed of cellulose.

The importance of plants for humans is indisputable. Without them we could not live, since the plants participated in the composition of the gases present in the Earth's atmosphere and in the ecosystems, and are the primary source of food for heterotrophic organisms. In addition, plants have direct importance to man: as a source of food; as building materials, firewood, fuel and paper; as ornamental; as substances that worsen or improve health and therefore have medical importance; and as a consequence of the latter, as the raw material of the pharmaceutical industry.

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.