The old couple walked rather slowly, and could be seen, from time to time, to stop and rest, out to sea.
A. to be staring
B. stared
C. having stared
D. staring
网友回答
A. to be staring
B. stared
C. having stared
(答案→)D. staring
解析：D 考察非谓语动词，stare的逻辑主语是the old couple，所以选择D，staring选择副词短语，在句中做伴随状语。