4. Determine how many different values of $a$ exist for which the equation

$$
\left(a^{2}-5\right) x^{2}-2 a x+1=0
$$

has a unique solution
Answer: 2. (C)

Options.

$$
\begin{array}{|l|l|l|l|l|l|l|l|l|}
\hline \mathbf{A} & 0 & \mathbf{B} & 1 & \mathbf{C} & 2 & \mathbf{D} & 3 & \mathbf{E} \\
\hline
\end{array}
$$