Suppose you have three variable resistors each of range 15 ohms to 30 ohms. Which of the following values of equivalent resistance cannot be obtained using these 3 variable resistors?

Question

$\begin {array} {1 1} (A)\;10\Omega & \quad (B)\;45\Omega \\ (C)\;61\Omega & \quad (D)\;100\Omega \end {array}$