`A B` is a double ordinate of the parabola `y^2=4a xdot` Tangents drawn to the parabola at `Aa n dB` meet the y-axis at `A_1a n dB_1` , respectively. If the area of trapezium `AA_1B_1B` is equal to `12 a^2,` then the angle subtended by `A_1B_1` at the focus of the parabola is equal to `2tan^(-1)(3)` (b) `tan^(-1)(3)` `2tan^(-1)(2)` (d) `tan^(-1)(2)`