Answer:
The factors that influence a speaker's or writer's choice of language vary, and they include the context that surrounds the speaker or writer, the age, gender, culture, etc. Very often, the choice of language is conscious, and the speaker can switch the language choice depending on such factors
I assume your choices are either social, informal, or business? I think it's social... it's definitely not business, and it doesn't sound informal.
Because they believe Sirius Black is there.
