Cotton grass, which grows only in arctic regions, has been the only summertime source of protein available to caribou. Caribou that do not get adequate amounts of protein in the summer are unable to reproduce the following year. Rising average temperatures in arctic regions, however, are causing cotton grass to disappear. Ttherefore, if the warming trend continues, caribou are likely to become extinct. Which of the following is an assumption on which the argument depends? ['Caribou that do not eat enough protein to reproduce do not live as long as caribou that do', 'The warming trend in arctic regions will not enable other plants capable of providing protein to caribou to grow there.', 'The caribou is the only animal that depends on cotton grass as a major source of food.', "Cotton grass is the only one of the caribou's food source that is becoming scarce as temperatures rise in arctic regions."]