Which of the following is an electrophile?

If $A_i$ $(i=1, 2, 3, \ldots, n)$ are $n$ independent events with $P(A_i) = \frac{1}{1+i}$ for each $i$, then the probability that none of $A_i$ occurs is

Which one of the following has the smallest diameter?

If the equation of one tangent to the circle with centre at $(2, -1)$ from the origin is $3x + y = 0$,then the equation of the other tangent through the origin is

If in a $\triangle ABC$,$\frac{1}{a+c} + \frac{1}{b+c} = \frac{3}{a+b+c}$,then $\angle C$ is equal to (in $^{\circ}$)

$A$ rectangular glass slab $ABCD,$ of refractive index $n_1,$ is immersed in water of refractive index $n_2$ $(n_1 > n_2)$. $A$ ray of light is incident at the surface $AB$ of the slab as shown. The maximum value of the angle of incidence $\alpha_{\max},$ such that the ray comes out only from the other surface $CD,$ is given by: