He walked down the hills and softly to himself.A. singB. singingC. singsD. was singing

问题补充：

He walked down the hills, and softly to himself.

A. sing

B. singing

C. sings

D. was singing

答案：

A. sing

B. singing

C. sings

(答案→)D. was singing

解析： 本句中的and是一个并列连词，前后都应该是谓语部分，AC两项的时态不对。B项不能单独做谓语。句义：他走下小山，同时在对自己唱歌。更加句义可知走下山和唱歌是同时发生的事情，故使用过去进行时。故D正确。