The JPS (1917) reads:
Therefore the L-rd Himself shall give you a sign: behold, the young woman shall conceive, and bear a son, and shall call his name Immanuel.
But the ESV reads:
Therefore the Lord himself will give you a sign. Behold, the virgin shall conceive and bear a son, and shall call his name Immanuel.
Which one of these is closest to the Masoretic Text (MT), and which is closest to the Septuagint (LXX)? How do textual critics decide which of these is the most likely reading?