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In this Aug. 16, 2019, photo, New York University student researchers sit on a rock overlooking the Helheim glacier in Greenland. U.S. President Trump announced his decision to postpone an early September visit to Denmark by tweet Tuesday Aug. 20, 2019, after Danish Prime Minister Mette Frederiksen dismissed the notion of selling Greenland to the U.S. as "an absurd discussion." (AP Photo/Felipe Dana)