Annie Jump Cannon, ______ discovered so many stars that she was called "the census taker of the sky".

A．a leading astronomer,

B．who, as a leading astronomer,

C．was a leading astronomer,

D．a leading astronomer who

大灾之后要防大疫．甘肃舟曲泥石流灾害后，救灾人员在灾区喷洒了大量消毒液．从预防传染病的角度分析，此举属于（　　）

A．控制传染源

B．切断传播途径

C．预防接种

D．保护易感人群