We find ______ impossible for us to learn a foreign language well in a short time.

. We called the First-Aid Center ____ thetraffic accident happened.

Most birds find it safe to sleep in the trees, but __________ they have eggs or young chicks, they don’t use a nest.

下列各句中加横线词语的使用，不正确的一项是（ ）

Tom’s parents are worried that he is__tothe computer games which is__to children.