The formula for finding the value of $D$ (discriminant) for a quadratic equation $ax^2 + bx + c = 0$ is $\ldots \ldots \ldots \ldots.$

• A
  $D = b^2 + 4ac$
• B
  $D = b^2 - 4a^2c^2$
• C
  $D = b^2 - 4ac$
• D
  $D = b^2 + 4a^2c^2$