单选题 Nobody yet knows how long and how seriously the shakiness in the financial system will the economy.
A. knock down B. drag down C. settle down D. put down

【正确答案】 B
【答案解析】drag down把…拖垮，身体垮掉。knock down拆散，拆毁，撞倒。settle down安顿下来。put down镇压，压制。