Which is the best argument for why CaBr2 represents an ionic 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 less than 1.7 but greater than 0.

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