观察下面一列数，按某种规律在横线上填上适当的数：﹣2，﹣4，0，﹣2，2，0，4，(      ),(      )

It’s necessary _________ us _________ more trees in spring every year.

A．of; to plant

B．for; to plant

C．of; planting

D．for; planting