The correct answer is - Continental polar air mass.
The continental polar air masses are very big air masses that are very cold, and dry. They are much heavier than the air masses that surround them, especially the ones that are further south of them, so they move with ease and push them away, thus spreading over a large territory very quickly and bringing in very cold and dry weather during the winter. This types of air masses are forming over Siberia , and the central part of Canada.
