一道英语问题一.词形变换.6.Oxygen can keep people (live) and (health).7.

6.Oxygen can keep people living and healthy.(keep doing或keep + 形容词)
7.We cut down and burn millions of trees every year.
8.Trees are out best fighting against pollution. 应该是fight的名词
9.In autumn leaves begin to fall down.
10.Can trees protect themselves against insects?
trees复数

6，living，healthing（keep后加名词的ing形式）
7，millions（of前的百，千，万都是加s）
8，fighting（前面有be动词are）
9，leaves（叶子的复数形式）
10，themselves（复数形式）