According to the Band Theory, Which of These Statements Provide(s)

According to the band theory, which of these statements provide(s) an explanation for the high electrical conductivity of metals
I. a partly filled conduction band
II. a valence band overlapping an empty conduction band
III. a filled valence band
IV. a large gap between the valence band and the conduction band

A) I and II
B) I and III
C) III
D) III and IV
E) IV