The Olympic Games are held _____.

A．for four years

B．each four year

C．every four years

D．every fourth year

锅炉的支出热量有：烟气带走的热量、灰渣(飞灰)带走的热量、炉壁散热损失的热量和燃料由于不完全燃烧未能发出的热量。