What is the primary significance of the prophecy in Isaiah 7:14 regarding a 'virgin' (almah) conceiving and bearing a son, Immanuel, in its immediate historical context?

A It was solely a prediction of the future birth of Jesus Christ and had no relevance to Isaiah's time. B It was a sign to King Ahaz of Judah, assuring him of God's protection from the immediate threat of the Syro-Ephraimite alliance. C It was a prophecy about the future destruction of the Assyrian empire. D It was a symbolic representation of the spiritual state of Israel at the time.