Answer: Because thear is only one parent.
Answer: One sign of physical change is melting, freezing.
The air that we breathe is mostly made up of nitrogen (78%). 21% is oxygen which is needed for animals and humans in respiration and 0.03% is carbon dioxide which is needed for plants to make energy in a process called photosynthesis. The bit that is left is made up of rare gases like helium and argon and methane.
Answer:
there are 4 major classes of biological macromolecules
