How Did Fra Andrea Pozzo Create the Illusion of Heaven

How did Fra Andrea Pozzo create the illusion of Heaven opening above the heads of the congregation in the church of Sant'Ignazio?

A) ​He illusionistically continued the church's architecture into the vault.
B) ​He renovated the nave ceiling to provide a better view of the paintings in the vault.
C) ​He painted quadro riportato to simulate a painted view of Heaven.
D) ​He illusionistically lowered the church's actual architecture.