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。