Which of the following gases present in the air darkens the surface painted with white lead?

In the given circuit, the current through the battery is ........ $A$.

If the sum of two of the roots of $x^3+p x^2-q x+r=0$ is zero,then $p q$ is equal to

Observe the following statements:
$1.$ Heavy water is harmful for the growth of animals.
$2.$ Heavy water reacts with $Al_4C_3$ and forms deuterated acetylene.
$3.$ $BaCl_2 \cdot 2D_2O$ is an example of interstitial deuterate.
The correct statements are:

If $200 \text{ MeV}$ of energy is released in the fission of one nucleus of ${}_{92}^{236}U$,the number of nuclei that must undergo fission to release an energy of $1000 \text{ J}$ is

Nitrobenzene on reduction using zinc in alkaline medium results in $X$. The number of $\sigma$ and $\pi$ bonds in $X$ is