Why is the first one (A) correct?

What diagram shows the ionic compound of magnesium oxide

The bond energy for the van der waals bond between two helium atoms is 7.9×10−4ev. assuming that the average kinetic energy of a helium atom is (3/2)kbt, at what temperature is the average kinetic energy equal to the bond energy between two helium atoms

When the mercury level in a barometer decreases that atmospheric pressure has