Vitamin D helps our bodies make a special chemical, and this special chemical, _______, affects our brains, making us feel happy.
A child who lives in a boring environment will develop his intelligence less than one who lives in rich and varied________.
The policemen soon came to the________where the accident happened.
They are________to use chemical fertilizers in the field by law.
They're trying to________waste discharged by the factory for profit.
The food wasn't good, but________it was cheap.