The correct answer is after the maximum number of days of snow cover.
In order to survive and reproduce, all the living species must amend to the situations imposed on them by their surroundings. A species environment includes everything imposing upon it, as well as everything, which is influenced by that species.
In the given case, white mice are found in tandem at the time of white snow cover, as they adapted themselves to the environment by changing their appearance from brown to white, so that they can camouflage themselves against the predators, like fox during winter times.