Given two coupled inductors $$L_1$$ and $$L_2$$ having their mutual inductance M. The relationship among them must satisfy

A

$$M > \frac{L_1 + L_2}{2}$$

B

$$M > L_1 L_2$$

C

$$M \leq L_1 L_2$$

D

$$M = L_1 L_2$$