Which of These Molecules Are Structural Isomers of N-Octane (C₈H₁₈)

Which of these molecules are structural isomers of n-octane (C₈H₁₈) ?

A) I, III, and IV
B) II and V
C) only II has 8 carbon atoms in a chain
D) III and IV
E) I, II, III, IV, and V