Only ______ as a translator ______ how important it was to grasp English.

A．when did I work; I realized

B．when did I work; did I realize

C．when I worked; I realized

D．when I worked; did I realize

预防和控制伤寒的主要措施是（）

A.检出带菌者，接触者进行医学观察

B.早期隔离和治疗患者

C.伤寒菌苗预防接种

D.疫区消毒

E.切断传播途径