Which of the following is not optically active?

In a sinusoidal wave,the time required for a particular point to move from maximum displacement to zero displacement is $0.170 \,s$. The frequency of the wave is ........ $Hz$.

To what temperature should the hydrogen at $327^{\circ}C$ be cooled at constant pressure,so that the root mean square velocity of its molecules becomes half of its previous value? (in $^{\circ}C$)

$A$ converging beam of rays is incident on a diverging lens. Having passed through the lens,the rays intersect at a point $15 \, cm$ from the lens on the opposite side. If the lens is removed,the point where the rays meet will move $5 \, cm$ closer to the lens. The focal length of the lens is.....$cm$

The instantaneous angular position of a point on a rotating wheel is given by the equation $\theta(t) = 2t^3 - 6t^2$. The torque on the wheel becomes zero at $t = $ ....... $\text{sec}$.

If $(1, 3), (2, 5),$ and $(3, 3)$ are three elements of $A \times B$ and the total number of elements in $A \times B$ is $6$,then the remaining elements of $A \times B$ are: