The compound butanol has the following structural formula. A string of 4 C atoms are bonded above, left, and below to H. The right-hand end is bonded to O, which in turn is bonded to H.

Which of these is a structural isomer of butanol?
A string of 4 C atoms are bonded above, below, left and right to H.
A string of 4 C atoms is bonded above, below, left, and right to H, except the second C, which is bonded below to O, which is bonded below to H.
A string of 4 C atoms is bonded above, below, left, and right to H, but the chain is interrupted between the first and second C, which are bonded to an O between them.
A string of 4 C atoms is bonded above, below, and left to H, except the last C has no H below and is double-bonded to an O to the right.