If the roots of the equations $px^2 + 2qx + r = 0$ and $qx^2 - 2\sqrt{pr}x + q = 0$ are real,then

Most suitable method of introducing alien $DNA$ into a plant cell is

$p$ points are chosen on each of the three coplanar lines. The maximum number of triangles formed with vertices at these points is

If the distance between the foci of an ellipse is $6$ and the length of the minor axis is $8$,then the eccentricity is

When radiation of wavelength $\lambda$ is incident on a metallic surface,the stopping potential is $4.8 \ V$. If the surface is illuminated with radiation of double the wavelength,then the stopping potential becomes $1.6 \ V$. The threshold wavelength for the surface is:

If the circle $x^2+y^2=a^2$ intersects the hyperbola $xy=c^2$ in four points $(x_i, y_i)$,for $i=1, 2, 3, 4$,then $y_1+y_2+y_3+y_4$ equals