People are using more and more natural gas, wind and other forms of _____.
His mother was born in Green, and Greek was her native language, ___.
While reading, you'd better make a mark ___ you have any questions.
Don't keep your mouth ___ if you want to improve your spoken English.
My classmates, Li Ming, was highly praised for ___ a good deed.
There are many pine trees on ___ of the street.