Matematically speaking, maybe because:
The number of substances = number of elements + number of different combinations of those elements
alumunum and oxygon with 2 Al atoms and 3 oxygen atoms make alumunum trioxide hope i went full god mode right there
Explanation:
It is given that r = 0.283 nm. As 1 nm =
.
Hence, 0.283 nm = 
- Formula for coulombic energy is as follows.

where, e =
C
= 

= 
- As 1 eV =

So, 1 J = 
Hence, U = 
= 8.9 eV
- Also, 1 J =

=
kJ/mol
Therefore, U =
kJ/mol
= 
Conduction - by touch
Convection - hot air rises, cold air sinks
Insulation - to insulate or capture heat
Radiation - by waves
Direct contact means touch, therefore the answer would be conduction.