After Christianity Received Acceptance Within the Roman Empire,churches Were Constructed

Multiple Choice

After Christianity received acceptance within the Roman Empire,churches were constructed above the catacombs.Which of the following provides the most explanation for such churches to be constructed at these sites?

A) The pious worshiped openly at the graves of the earliest Christian martyrs
B) These were the only sites available for church construction
C) All churches had to be constructed outside the city's walls
D) The catacombs were the holiest sites