The United Nations Educational, Scientific, and Cultural Organization's Model of Schooling

The United Nations Educational, Scientific, and Cultural Organization's model of schooling that most countries have adopted includes:

A) 6 years of primary school and 3 years of secondary school
B) 6 years of primary school with the option of secondary school
C) 8 years of primary school with 4 years of secondary school
D) 8 years of primary school with the option of secondary school