A would be correct. Hope i helped.
Answer:
The mass number tells us the number (the sum of nucleons) of protons and neutrons in the nucleus of an atom. The atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom. It is traditionally represented by the symbol z
Explanation:
Answer:
gas
Explanation:
It is a gas because it can't be seen for long or at all in the air.
The main job of these systems is to break down food and for it to exit the body after taking out of it what is necessary such as nutrients