A：动物个体
B：植物个体
C：表皮
D：细胞
空气是由多种气体组成的混合物，是一种宝贵的自然资源．
The question of __________ English will keep on changing in the future is easy __________ since it is certain that this process will continue.
“千里之堤，溃于蚁穴”说明（ ）
