Q: 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: The history of a unified Tibet begins with the rule of Songtsan Gampo (604650 CE), who united parts of the Yarlung River Valley and founded the Tibetan Empire. He also brought in many reforms, and Tibetan power spread rapidly, creating a large and powerful empire. It is traditionally considered that his first wife was the Princess of Nepal, Bhrikuti, and that she played a great role in the establishment of Buddhism in Tibet. In 640 he married Princess Wencheng, the niece of the powerful Chinese emperor Taizong of Tang China.
A:
Who ruled Tibet from 640-650 CE?