The most important thing _____ we should consider is the first idea ____ he has talked about in the speech. [ ]

A. which; that

B. that; which

C. which; which

D. that; that

阻塞高压是中纬度高空（）的（）高压系统，它的东西两侧盛行（）向气流，高压的南侧有明显的（）风。