．It was the four years from 2004-2007, ____ I spent studying for a master’s degree, ____ I will never forget．

A．when; that

B．when; when

C．that; that

D．which ; that

某油库工人许某，在上班进行输油作业过程中擅离职守，离开作业现场。储油罐装满后，汽油外溢，库区地面漂流大量汽油，空气中也形成大量可燃气体，因遇输电线火星；发生严重火灾，整个油库成为一片火海，给国家造成巨大经济损失，许某的行为构成：( )

A．失火罪

B．重大责任事故罪

C．玩忽职守罪

D．危险品肇事罪