A male mouse housed alone in a home cage will readily attack a male mouse placed into the home cage, ...

A male mouse housed alone in a home cage will readily attack a male mouse placed into the home cage, but will not attack a female mouse.
 
  Which of the following is a key factor as to whether the resident male mouse decides to attack or not attack the strange male mouse?
 
  a. Male mice are smaller than females (thus size is the issue).
  b. The mouse uses ultrasounds to distinguish male from female.
  c. Female mice show a hop-dart movement that male mice do not.
  d. The mouse must be able to detect male pheromones.
  e. The coat color of a female mouse is lighter than that of a male mouse.