English single choice

The first two questions examine the usage of it in referring to people. Refers to a person, mainly used to refer to a baby with unknown sex or to confirm someone's identity. Someone knocked at the door. It must be the postman. Someone is knocking at the door. It must be the postman. The third question examines the usage of it as a non-personal pronoun. The usage of it as a non-personal pronoun mainly refers to natural phenomena such as time, distance, value, weather, climate and temperature. It rained all day yesterday. It rained all day yesterday.