In their native habitat, amaryllis plants go dormant when the soil in which they are growing dries out during the dry season. Ttherefore, if amaryllis plants kept as houseplants are to thrive, water should be withheld from them during part of the year so that the plants go dormant. Which one of the following is an assumption on which the argument depends? ['Most kinds of plants go dormant at some time or other during the year.', 'Amaryllis are more difficult to keep as houseplants than other kinds of plants are.', 'Any amaryllis plant that fails to thrive is likely to have been dormant for too short a time.', 'Going dormant benefits amaryllis plants in their native habitat in some way other than simply preventing death during overly dry periods.']