The correct answer is - Continental Polar air masses.
The Continental Polar air masses which are dry and very cold, are forming above most of what is the mainland of the country of Canada. These air masses are able to reach the Gulf of Mexico, and thus Texas, with ease, and very quickly, because they can move freely through the plains that stretch through the central part of the continent and there's no obstacle that will stop them or make them move to another direction. The Continental Polar air masses bring sudden cold and dry weather, and they are the most active from late autumn to early spring, being the most intense during the winter.
