A、The west coast. B、The Arctic coast. C、Southern Ontario. D、Atlantic Canada. A文章的第一句话就直接指出：加拿大西海岸的气候最温和(the most moderate)，所以选A

The climate of the west coast is the most moderate in Canada. Summers are cool and fairly dry and winters are mild, cloudy and wet. Even in mid-winter, average temperatures are usually above freezing.
    The central plain from the Rocky Mountains to Great Lakes is characterized by cold winters, short but hot summers, and light snow and rain. Tile large water-surfaces of Central and Eastern Canada produce considerable modification in the climate. Southern Ontario and Quebec experience cold, damp winters and hot, humid summers.
    Most of Atlantic Canada has a humid climate owing to its maritime character. Nevertheless, it experiences weather systems arriving from the dry, continental interior as well as from the sea. The combined influence of these systems creates some of the most variable day-to-day weather conditions to be encountered anywhere in Canada.
    The north-central part of Canada is usually snow-covered for more than half of the year, with a frost-free period of barely two months. Rain is relatively light. Further north, on the islands along the Arctic coast and round Hudson Bay, the land. is always frozen. Average temperatures stay above freezing for only a few weeks of the year. The Arctic Islands and the northern border of the mainland do not have a summer season of the kind known in Southern Canada.