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Luke 24:50-1 When he had led them out to the vicinity of Bethany, he lifted up his hands and blessed them. While he was blessing them, he left them and was taken up into heaven.
Mark 16:14, 19 Later Jesus appeared to the Eleven as they were eating; he rebuked them for their lack of faith and their stubborn refusal to believe those who had seen him after he had risen...After the Lord Jesus had spoken to them, he was taken up into heaven and he sat at the right hand of God.
Acts 1:9-12 After he said this, he was taken up before their very eyes, and a cloud hid him from their sight. They were looking intently up into the sky as he was going, when suddenly two men dressed in white stood beside them. "Men of Galilee," they said, "why do you stand here looking into the sky? This same Jesus, who has been taken from you into heaven, will come back in the same way you have seen him go into heaven." Then they returned to Jerusalem from the hill called the Mount of Olives, a Sabbath day's walk from the city.
Some critics think these verses provide contradictory locations for the Ascension.
We have noted that Mark 16:8ff is not to be taken into consideration.
As for the other two cites, Bethany is located on the slope of the Mount of Olives, so the verses are in agreement; they merely use different geographical referents to establish the location. (Note that Luke's gospel refers to the vicinity of Bethany, whereas Acts refers specifically to the Mount of Olives.)