UN Convention on Contracts for the International Sale of Goods -irrevocab Offers

Yeo sends a written offer to Zal stating that the offer will remain open until 14 July. The offer
reaches Zal on 7 July, but on 12 July Yeo decides that he no longer wishes to be held to the offer.
Under the UN Convention on Contracts for the International Sale of Goods, which of the
following applies to Yeo?
A He can withdraw the offer but cannot not revoke it
B He can revoke the offer but cannot withdraw it
C He can withdraw the offer or he can revoke it
D He cannot withdraw the offer nor can he revoke it

The correct answer is D, as according to the explanation provided, the offer is irrevocable. However, according to the text provided, which part of it implies the irrevocability element of the offer?
Thanks.