If the unit cell of a mineral has cubic close packed (ccp) array of oxygen atoms with m fraction of octahedral holes occupied by aluminium ions and n fraction of tetrahedral holes occupied by magnesium ions, m and n, respectively, are

A

$$\frac{1}{2}, \frac{1}{8}$$

B

$$1, \frac{1}{4}$$

C

$$\frac{1}{2}, \frac{1}{2}$$

D

$$\frac{1}{4}, \frac{1}{8}$$