The One Important Requirement Often Missing from These Church Interiors

The one important requirement often missing from these church interiors is light.Which of the following did not allow for interior lighting?

A) Barrel-vaults exerted great outward thrust making a clerestory difficult to construct
B) Light not considered an important spiritual ingredient for church services
C) The massive walls would not support a tribune and clerestory
D) It was considered too expensive to fund the construction of a clerestory