下列选项中，最能说明我国经济重心从黄河流域转移到长江流域的是[ ]

A．辽、西夏、金学习汉族先进文化

B．“苏湖熟，天下足”

C．南宋临安人口超过百万

D．元朝统一

From the ___ expression on her face, we knew that the problem was ____.

A．puzzling….puzzling

B．puzzled…puzzled

C．puzzling..puzzled

D．puzzled..puzzling