If the average of n quantities is P and the average of m quantities is Q, then the average of (m + n) quantities is:

A

$$\frac{nP+mQ}{m-n}$$

B

$$\frac{mP+nQ}{m+n}$$

C

$$\frac{nP+mQ}{P+Q}$$

D

$$\frac{nP+mQ}{m+n}$$