How did the invention of the barrel vault contribute in the construction of churches?

It was extremely difficult to construct massive churches because of the huge outward thrust against the walls which exists in all arch based structures. When the barrel vault was invented, they would place two or more barrel vaults parallel and the outward thrust of the vaults would negate each other bringing stability. Later they became groin vaults when people found out that they could be placed in an intersection.

Your answer would be a in this question good luck