Pick out the correct statement from the following:

$A$ neutron collides head-on with a stationary hydrogen atom in its ground state.

The crystal structure of $NaCl$ is:

$A$ photon collides with a stationary hydrogen atom in the ground state inelastically. The energy of the colliding photon is $10.2 \ eV$. After a time interval of the order of a microsecond,another photon collides with the same hydrogen atom inelastically with an energy of $15 \ eV$. What will be observed by the detector?

The number of completely filled shells for the element ${ }_{16} S ^{32}$ is

The wavelength of the emission line spectrum and the absorption line spectrum of a substance are related as: