吸烟可致肺癌但不是每个吸烟者都会得肺癌，即吸烟者肺癌发生率＜100%；另外不是每个肺癌病人以前都吸烟，即肺癌病人以前的吸烟率＜100%，此实例的病因组合符合

A．充分而且必要病因

B．充分但不必要病因

C．不充分且不必要病因

D．必要但不充分病因

E．以上都不是

According to the school rules, no child _______ be allowed out of the school during the day, unless accompanied by an adult．

A．will

B．would

C．might

D．shall