Ans. a. EnvironmentalA girl and 40% of her classmates are suffering from white hair. Her brother who is staying with an aunt in the neighboring village is normal and does not have white hair. In the given question, as the girl and many of her classmates (from a single geographical area, in this case, the school) have developed grey hair, the most likely cause would be environmental.Similarly, as many other children from the same school have white hair, the cause is also unlikely to be nutritional.As the girl's brother living in a different village does not have white hair, the cause of the white hair is unlikely to be genetic