Given the task definition and input, reply with output. This task is about creating an unanswerable question based on a given passage. Construct a question that looks relevant to the given context but is unanswerable. Following are a few suggestions about how to create unanswerable questions:
(i) create questions which require satisfying a constraint that is not mentioned in the passage
(ii) create questions which require information beyond what is provided in the passage in order to answer
(iii) replace an existing entity, number, date mentioned in the passage with other entity, number, date and use it in the question
(iv) create a question which is answerable from the passage and then replace one or two words by their antonyms or insert/remove negation words to make it unanswerable.

Passage: In Article 3 of the Treaty of San Francisco between the Allied Powers and Japan, which came into force in April 1952, the U.S. put the outlying islands of the Ryukyus, including the island of Okinawahome to over 1 million Okinawans related to the Japaneseand the Bonin Islands, the Volcano Islands, and Iwo Jima into U.S. trusteeship. All these trusteeships were slowly returned to Japanese rule. Okinawa was returned on May 15, 1972, but the U.S. stations troops in the island's bases as a defense for Japan.
When did Article 2 of the Treaty of San Francisco come into effect?