What might be the reasons why a European crown became a huge dispute or war among royal families at home and abroad when a king died and an heir or a new king was to be recognized in many caese?
选项：

A: Christianity allows Christians including kings to have only one wife.
B: When a king died and left no children, his cloest realtive would be the heir.
C: In old Europe, a member of a royal family usually married only a member of another royal family.
D: All lords and knights all wanted to become the king.