Answer: D.two or more elements are chemically bonded
Explanation:
Compound is a pure substance which is made from atoms of different elements chemically combined together in a fixed ratio by mass.It can be decomposed into simpler constituents using chemical reactions. They have different properties as compared to their constituents. Example: water 

Element is a pure substance which is composed of atoms of similar elements.It can not be decomposed into simpler constituents using chemical reactions.Example: Copper 
Thus every compound is made up of two or more elements chemically bonded.
The layer of skin that contains nerves and blood vessels is the______
Answer: Dermis
If you use a vacuum chamber as an example. When you pump out all the air from a chamber, all the molecules that normally push down on you, are gone, so molecules that are inside the vacuum start to move quicker.
3 lithium ions bond to one nitrogen ion.