$$\frac{1}{3\times3} + \frac{1}{4\5} + \frac{1}{5\6} + ----+ ----------- \frac{1}{20\21}$$
On simplification, the given expression will give the result as:

A

$$\frac{1}{20}$$

B

$$\frac{2}{7}$$

C

$$\frac{3}{7}$$

D

$$\frac{1}{7}$$