目前，中国粮食单产水平与世界粮食高产国家相比是低的。

A.At present, China's per unit area yield of grain is low compared to the countries with high grain yield.

B.At present, China's per unit area yield of grain is low compared the countries with high grain yield.

C.At present, China's per unit area yield of grain is low compared with the countries with high grain yield.