The answer is A. Ne. You can separate the elements in the other three choices through chemical changes (dissociation, ionization, electrolysis, etc.), but in order to separate the components of Ne, you would need a nuclear reaction (to decompose the nucleus) or a physical change (to strip the nucleus of its electron cloud).
Answer:

Explanation:
Hello,
In this case, considering that the density is considered as the quotient of the mass and volume:

We can easily compute the mass by solving for it as follows:

Best regards.
The independent variable is what you are testing in the experiment - it sounds like the independent variable in this experiment is the amount/concentration of nitrogen
Ionic bonds result when electrons are tranfered between atoms. Covalent bonds result when valence electrons are shared between atoms. You should be able to identify whether an ionic or covalent bond is being formed between atoms.