It is possible,and sometimes necessary, to strengthen the deck of a vessel for carriage of deck cargo by（）.

A.placing bunker on the deck

B.building a stage on which to place the cargo

C.welding steel feet to the deck, on which the cargo is placed

D.erecting vertical pillars under the deck to support the cargo

下图为某大陆多年平均1月、7月海平面气压（单位：百帕）分布图。读图，导致该大陆a月、b月气压状况不同的主要原因是（）

①海陆热力性质差异

②太阳直射点的回归运动

③地形起伏

④洋流分布

A、①②

B、②③

C、①④

D、③④