To What Does the Term the "Hidden Curriculum" Refer

To what does the term the "hidden curriculum" refer?

A) The standards of behaviour that are deemed proper by society and are taught subtly in schools.
B) The teaching of art, music, industrial arts, acting, and other nonessential courses in high schools.
C) The practice of placing students in specific curriculum groups on the basis of test scores and other criteria.
D) The lesson plans that instructors prepare and that students and parents never see.