Only when he got on the train _________ that he had left his mobile phone at home．

A．he realized

B．he had realized

C．had he realized

D．did he realize

关于水痘的流行病学特征，以下描述正确的是（）

A.病后免疫力可持续5～10年

B.感染者以儿童为主

C.现症病人与病毒携带者是传染源

D.以秋冬季节多发

E.传染性很强