Which of the following secretes interferons?

Drops of liquid of density $d$ are floating half immersed in a liquid of density $\rho$. If the surface tension of the liquid is $T$,then the radius of the drop is

Starting from rest,a body slides down a $45^\circ$ inclined plane in twice the time it takes to slide down the same distance in the absence of friction. The coefficient of friction between the body and the inclined plane is

$A$ particle falls from a height $h$ upon a fixed horizontal plane and rebounds. If $e$ is the coefficient of restitution,the total distance travelled before rebounding has stopped is

Dividing the interval $[0,6]$ into $6$ equal parts and by using the trapezoidal rule, the value of $\int_0^6 x^3 dx$ is approximately:

Two identical flutes produce fundamental notes of frequency $300 \ Hz$ at $27^{\circ}C.$ If the temperature of air in one flute is increased to $31^{\circ}C,$ the number of beats heard per second will be