Four pairs of metallic elements are listed with their atomic radii and crystal structures.
Pair I Cu (128 pm, FCC) and Ni (125 pm, FCC)
Pair II Cu (128 pm, FCC) and Zn (134 pm, HCP)
Pair III Ag (144 pm, FCC) and Mg (160 pm, HCP)
Pair IV Na (186 pm, BCC) and K (227 pm, BCC)
Which pair is most likely to exhibit extensive substitutional solid solubility over the entire composition range?
(a) Pair III
(b) Pair II
(c) Pair I
(d) Pair IV