Solid $NaCl$ is a bad conductor of electricity since

An object $2.4 \; m$ in front of a lens forms a sharp image on a film $12 \; cm$ behind the lens. $A$ glass plate $1 \; cm$ thick,of refractive index $1.50$ is interposed between the lens and the film with its plane faces parallel to the film. At what distance (from the lens) should the object be shifted to be in sharp focus on the film (in $; cm$)?

$A$ fresh radioactive sample is given at $t = 0$. Its decay fraction is $\frac{1}{5}$ at $t_1$ instant and $\frac{4}{5}$ at $t_2$ instant. Its mean life is

$SO_{2}$ is a strong reducing agent; however,it can also act as an oxidising agent. Which of the following shows its oxidising nature?

$A$ polygon has $54$ diagonals. Then,the number of its sides is

Which scientists conducted in-depth research in the field of plant classification?