The relation between dynamic plate resistance (rp­) of a vacuum diode and plate current in the space charge limited region, is

• A.            \[{{r}_{p}}\propto {{I}_{p}}\]
• B.            \[{{r}_{p}}\propto I_{p}^{3/2}\]
• C.            \[{{r}_{p}}\propto \frac{1}{{{I}_{p}}}\]                         
• D.            \[{{r}_{p}}\propto \frac{1}{{{({{I}_{p}})}^{1/3}}}\]