14) B

The two spikes at 35 and 37 indicate two isotopes of element X.

The spike at 70 is due to two isotopes of mass 35 existing as a diatomic molecule. (35 + 35 = 70)

The spike at 72 is due to two isotopes of mass 35 and 37 existing as a diatomic molecule.
(35 + 37 = 72)

The spike at 74 is due to two isotopes of mass 37 existing as a diatomic molecule. (37 + 37 = 74)

So B is the answer.