Stress: refers to the response of all organisms to various external stimuli. It is a dynamic response and is completed in a relatively short period of time. According to the above definition, which of the following is not stress? ['Yellow weasel emits an odour when it encounters enemy.', 'Various reactions when people see text and images.', "Polar bear's fur is white, consistent with the colour of the environment in which it lives.", 'Cicada generally does not tweet below 24 ℃, but only tweet when the temperature is above 24 ℃ and the light reaches a certain level.']