How Would Crowded Conditions in Cities Favor Spread of Plague

How would crowded conditions in cities favor spread of plague?

A) Plague is transmitted by rats-more people means more waste,and more waste means more rats.This would favor the spread of plague.
B) Plague is transmitted by mosquitoes-more people close together gives an infected mosquito more chances to bite humans and transmit the causative agent,spreading plague.
C) Plague is transmitted by infected fleas-these fleas may be found on rodents (such as rats) .More people in an area means a greater chance of interactions with animals carrying infected fleas,increasing the spread of plague.
D) Plague is transmitted by direct contact (e.g.skin to skin) .More people in an area provides more chances for infected individuals to directly contact and infect other individuals,spreading plague.