Although diamonds and graphite both consist of the element carbon, their physical properties are very different. The most likely explanation for these differences is that:
a) Diamonds have a cubic crystal structure, while graphite has a hexagonal crystal structure
b) Diamonds are formed at high pressures and temperatures, while graphite is formed at lower pressures and temperatures
c) Diamonds have a covalent bonding structure, while graphite has a layered structure held together by weak Van der Waals forces
d) Diamonds have a higher density, while graphite has a lower density