It is necessary for a coordinate to be in quadrant III that both x and y coordinate have to be negative. For e.g (-2,-2) have both x and y coordinate negative , so it is in quadrant III , where as (2,-2) or(-2,2) are not in quadrant III. In the given question C (-a,-a) have both x and y coordinates negative. SO C is in third quadrant and the correct option is C .
