In a few years` time,millions of trees those mountains

题目

In a few years` time,millions of trees those mountains
In a few years` time,millions of trees _________（cover） those mountains
为什么不能是have covered

答案

is covering
因为树始终还是覆盖这山
而have covered说明树以前是覆盖的
但现在不再覆盖山了