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: Elizabeth's personal fortune has been the subject of speculation for many years. Jock Colville, who was her former private secretary and a director of her bank, Coutts, estimated her wealth in 1971 at 2 million (equivalent to about 25 million today). In 1993, Buckingham Palace called estimates of 100 million "grossly overstated". She inherited an estimated 70 million estate from her mother in 2002. The Sunday Times Rich List 2015 estimated her private wealth at 340 million, making her the 302nd richest person in the UK.
In what year did Jock Colville become Elizabeth's private secretary?