Which is the best argument for why CO2 represents a covalent bond?

a
The difference in electronegativity of the two elements is greater than 1.7.

b
The difference in electronegativity of the two elements is less than 1.7.

c
The difference in electronegativity of the two elements is greater than 1.7 but less than 3.0.

d
The difference in electronegativity of the two elements is zero.