Which of the following is/are barbiturates?

The mean of four observations is $3$. If the sum of the squares of these observations is $48$,then their standard deviation is

Consider the following two statements $A$ and $B$ and identify the correct choice in the given answers:
$A$. Paramagnetism is explained by domain theory.
$B$. Susceptibility of a diamagnetic substance is independent of temperature.

$A$ black body at a temperature of $1640\,K$ has the wavelength corresponding to maximum emission equal to $1.75\,\mu m$. Assuming the moon to be a perfectly black body,the temperature of the moon,if the wavelength corresponding to maximum emission is $14.35\,\mu m$,is.......$K$.

Two similar plano-convex lenses are combined together in three different ways as shown in the figure. The ratio of the focal lengths in the three cases will be

The condition that the roots of $x^3-b x^2+c x-d=0$ are in geometric progression is