Global warming. Not sure if that's one of the answer choices or if it's more technical than that.
The great migration between 800 and 1200 saw people of khmer and mon ethnicity move into the northern region of modern day Myanmar - D.
Following this, they were able to survive and live were, although not without great difficulties.
This would be the definition of an obligation.
I believe the answer is: only legal citizens explains the need for the 14th amendment
The 14th amendment dictate how the government could not override the natural and legal rights that owned by the citizen of united states. This protection is only provided for legal citizens of united states and not extended to other people in the country (diplomats and illegal immigrants)