Vitamin D helps our bodies make a special chemical, and this special chemical, _______, affects our brains, making us feel happy.
--Would you like to play basketball with me? --________, but I have too much homework to do.
What impressed me most was that they never ________.
A meeting was held ______ those who lost their lives in the fight against SARS.
The Chinese teacher often requires the students to talk about writers and books________they like most.
John is a creative person. He can often_____ideas that are surprising to his classmates.