Illegal Forward Pass?

[dldsooner]

In last nights meeting we were taking a quiz, and one question in particular is bothering me.
On first and 10, Team A's QB runs beyond the neutral zone, retreats behind the neutral zone, and throws a forward pass from his 40 yard line. B37 interferes with eligible A86 who catches the pass and runs for an apparent touchdown.

(a)double foul for A's illegal forward pass and B37's pass interference.
(b)Team A is penalized five yard from the 40 yard line plus loss of down; second down from the 35 yard line.
(c)the clock starts on the snap
(d)No foul by B37; defensive pass interference cannot be called on an illegal forward pass play
(e)The penalty is obviously declined.
(f)None of the above

Any and all of these answers can either be correct or incorrect.
This was taken out of an officials book that the president of our association has. According to the book the correct answer is---F---.

Now I always thought that once the QB crossed the line of scrimmage it did not matter what he did, if he crosses back behind the neutral zone and throws a forward pass, it is an illegal forward pass.
We argued over this for 30 minutes last night, and could not find anything in our rule book or case book, if anyone has anything to clear this up I would appreciate it.