Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is

The oxide of nitrogen formed by thermal decomposition of $NH_4NO_3$ is

Given below is a highly simplified representation of the human sex chromosomes from a karyotype. The genes 'a' and 'b' could be of

$\cos 36^{\circ} - \cos 72^{\circ}$ is equal to

In a mass spectrometer used for measuring the masses of ions,the ions are initially accelerated by an electric potential $V$ and then made to describe semicircular paths of radius $R$ using a magnetic field $B$. If $V$ and $B$ are kept constant,the ratio $\left( \frac{\text{charge on the ion}}{\text{mass of the ion}} \right)$ will be proportional to

Which one of the following represents the correct order of electronegativity?