One of the key duties of a security guard is to ensure the safety of the building.

duty 是名词，意思是"责任、职责"，来自古法语"deu"（应得的）和拉丁语"debere"（欠债），引申为"应尽的义务"。 duties 是 duty 的复数形式，表示"多项职责"。