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1. Correct. Though which looks like a wrong GMAT usage, everything else is correct.
2. Wrong. Ambiguous "they" in sentence ?? I do not see clarity in what exactly "they" refers to.
3. Wrong. "would be mountain dwellers" is incorrect.
4. Wrong. Modifier should be Elk. The "were mountain dwellers" giving it a meaning of the past.
5. Wrong. Wordy and awkward.