Vitamin D helps our bodies make a special chemical,and this special chemical,________,affects our brains,making us feel happy.
They kept trying _____ they must have known it was hopeless.
An awful accident, however, _____occur the other day.
Little Jim is going to spend his holiday in Paris, _____ lives his uncle.
The person we referred to(提及)____ us a report tomorrow.
The research is so designed that once _____ nothing can be done to change it.