withdraw of an offer

According to this question I want to the right answer.
According the UN Convention on Contracts for the International Sale of Goods, what is the latest point in
time that an offer may be withdrawn?

Answer is written b the book like that, Under UNCISG, an offer may be withdrawn at any time up to when the offer is delivered to the
offeree.

But answer is chosen bu this option : At the same time the offer reaches the offeree

Therefore, I want to clarify at any time or at the same time?