Read this and answer the question. If the question is unanswerable, say "unanswerable".

The earliest Tibetan historical texts identify the Zhang Zhung culture as a people who migrated from the Amdo region into what is now the region of Guge in western Tibet. Zhang Zhung is considered to be the original home of the Bön religion. By the 1st century BCE, a neighboring kingdom arose in the Yarlung valley, and the Yarlung king, Drigum Tsenpo, attempted to remove the influence of the Zhang Zhung by expelling the Zhang's Bön priests from Yarlung. He was assassinated and Zhang Zhung continued its dominance of the region until it was annexed by Songtsen Gampo in the 7th century. Prior to Songtsän Gampo, the kings of Tibet were more mythological than factual, and there is insufficient evidence of their existence.

Who annexed the Zhuang Zhung in the 1st century BCE?
unanswerable