28. The book was written _____ careful observations and detailed studies.
Many new means__________ to prevent the river ____________.
I like _____ in the spring when the weather is warm and everything begins to grow.
Is it to hold the 2010 World Exhibition _______ makes Shanghai the focus of the world?
We can never expect________bluer sky unless we create___________less polluted world.
Mike ______ have known the solutions to this problem, or he would have told us.