Difference between diamond and graphite is due to:
(A) Graphite combines with oxygen to form carbon dioxide but diamond does not.
(B) The atoms in each have different masses
(C) The crystal structure in diamond is different from that in graphite
(D) All

Hint:Diamond and graphite are the allotropic forms of carbon. Allotrope is the property of any element to exist in two or more different forms. In allotropes, there are different structural modifications of an element.

Complete step by step answer:
- We know that graphite and diamond are the allotropes of carbon. So, diamond and graphite chemically consist of carbon atoms and they have different physical properties.
- In both these structures, carbon atoms are bound together by strong covalent bonds.
- Diamond is a precious stone or it is known to be the hardest substance on the Earth whereas, Graphite is widely used in lead pencils. It is interesting to know that graphite and diamond have shift structures which represent their diverse properties.
- To answer this question, we should acknowledge that in diamond C atom is $s{{p}^{3}}$ hybridised due to tetrahedral structure, whereas, Graphite has planar hexagonal layers of carbon atoms that are held together by weak Van der Waals forces and C is $s{{p}^{2}}$ hybridised. So, from this we can say that the difference between diamond and graphite is due to the difference in crystal structure of diamond and graphite.
- The carbon atoms in graphite join to the three atoms of carbon and form plates like structures that are parallel to each other. Diamond has a structure in which the four atoms of carbon and make a tetrahedron structure.
-Difference in crystal structure of diamond and graphite is the reason that properties of diamond and graphite are different.

Hence option C is correct.

Note:
Do not get confused between allotropes and isotopes. Two isotopes have different atomic weight but allotropes always have the same atomic weight as they contain the same number of protons and neutrons in the nucleus.