Schools in the east tend to be better equipped those in the west are relatively poor.A.

问题补充：

Schools in the east tend to be better equipped, those in the west are relatively poor.

A. As

B. While

C. When

D. But

答案：

A. As

(答案→)B. While

C. When

D. But

解析：考查连词。While然而；句意：东部的学校有更好的设备，然而西部的学校却相贫穷。