当转化原料突然中断时，以下哪种操作是不允许的（）。

A.降低瓦斯入炉量，防止超温

B.继续通入蒸气

C.打通循环流程

D.切断入炉蒸汽

He had to speak in a loud voice in order to make himself_________.

A．hear

B．hearing

C．heard

D．to be heard