单选题
When ______ about substances, the scientists find it convenient to classify them into solid, liquid and gas.
A、
thought
B、
to think
C、
thinking
D、
having thought
【正确答案】
C
【答案解析】
现在分词thinking表主动；when thinking about substances相当于时间状语从句when they are thinking about substances。
提交答案
关闭