It seems that living green is __________ easy and affordable. A small step makes a big difference.
Mr. Herpin is one of the foreign experts who ___in China.
Anyone can borrow books from this library _______ he keeps them clean and returns them in time.
So fast _____that it is difficult for us to imagine its speed.
Just as the tree is the unit of a forest and the book the unit of a library, ______ the molecule is the unit of matter.
The door opened and in ________.