A：在通常情况下，为气体
B：能在空气中燃烧，燃烧时冒浓烟
C：能够使酸性高锰酸钾溶液褪色
D：在通常状况下，易溶于水
When he was running after his brother, the boy lost his ___ and had a bad fall.
碳元素与氧元素的本质区别是（ ）
After you learn about the lives of the pilots, especially their hard training, you can decide ______ you want to become a pilot or not.
