When Do Scientists Think the Caloris Basin Was Formed, and Why

Multiple Choice

When do scientists think the Caloris Basin was formed, and why do they think this?

A) It must have been formed early in Mercury's history, because it is overlapped by many other craters, some of them quite large.
B) It must have been formed early in Mercury's history, because the shock wave that created the jumbled terrain on the opposite side of the planet could only pass through Mercury when it was in a semi-molten state.
C) It must have been formed about the middle of the Heavy Bombardment period, because the crater density within the basin is only about half the average crater density for Mercury.
D) It must have been formed at the end of the Heavy Bombardment period, because the crater density in the basin is very low compared to the average crater density for Mercury.