Why Do Yellow-Bellied Marmot Females Live Two to Five Per

Why do yellow-bellied marmot females live two to five per den? Wouldn't it be better for them to each have their own den?

A) One den per female might be better, but there aren't enough dens to go around.
B) One female can't care for a litter of offspring by herself, so they have to live together and cooperate.
C) One male can mate with two to five females, so he will work to defend one den for that many.
D) The only way a female can defend a den from predators is if she has help.
E) There are more females than males, so this is the only way everyone gets to mate.