this后面加什么区别

例句：

This will be a problem for some time to come.

这将是未来一段时期里的.一个问题。

This is the one major drawback of the new system.

这是新系统的一大弊端。

This is one of the most commonly used methods.

这是最常采用的方法之一。

It's about this high.

大约有这么高。

The number of new students decreased from 210 to 160 this year.

今年新生人数从210减少到160。