He was walking along the sands he saw a big footprint in the sand.A. while B. asC. whenD.

问题补充：

He was walking along the sands he saw a big footprint in the sand.

A. while

B. as

C. when

D. after

答案：

A. while

B. as

(答案→)C. when

D. after

解析：考查固定句型。固定句型：be doing sth when…正在做某事在这时…（发生了意料之外的事情）；句意：他正在沙地上走着，在这时他看见了一个大脚印。故C正确。