The time and effort he has devoted during the past few years _____ trees in that village are nowconsidered to be very important. [ ]

____when has the country been open to international trade?—1978,I suppose

His expression suggested that he _______ ill, so the teacher suggested that he _______ sent to hospital at once.

This kind of material can be used to plastics.

下列有关文化常识的表述，不正确的一组是（ ）