Which of the following is an insecticide?

Assertion $(A)$ The effective resistance of the network between $P$ and $Q$ is $\frac{4}{5} r.$

Reason $(R):$ Symmetry can be applied to be network with respect to centre.

Two capacitors of capacities $1 \mu F$ and $C \mu F$ are connected in series and the combination is charged to a potential difference of $120 \ V$. If the charge on the combination is $80 \mu C$,the energy stored in the capacitor of capacity $C$ in $\mu J$ is

The sum of the last eight coefficients in the expansion of $(1 + x)^{15}$ is:

The radioactive sources $A$ and $B$ have half-lives of $2 \ hr$ and $4 \ hr$ respectively and initially contain the same number of radioactive atoms. At the end of $2 \ hr$,the ratio of their rates of disintegration is:

What is the use of potassium dichromate $(K_2Cr_2O_7)$?