Examine the following three pairs of possible isomers. Now state whether the pairs represent identical compounds or different isomers.

• A
  All three pairs represent different compounds
• B
  $Ia$ and $Ib$ are identical; $IIa$ and $IIb$ are identical; and $IIIa$ and $IIIb$ are identical
• C
  $Ia$ and $Ib$ are isomers; $IIa$ and $IIb$ are identical; and $IIIa$ and $IIIb$ are isomers
• D
  $Ia$ and $Ib$ are identical; $IIa$ and $IIb$ are identical; and $IIIa$ and $IIIb$ are isomers