The sign means we should not (litter/littering)应该选什么?

问题描述:

The sign means we should not (litter/littering)应该选什么?

should/would等情态动词后接动词原形,所以是litter。
一般口语表述为NO Littering!

litter

litter
should是情态动词所以后面用动词原形