Since the distance is always positive, speed is also positive.
One of the following does not apply to a concave mirror. This is : (a) focal length is negative (b) image distance can be positive or negative (c) image distance is always positive (d) height of image can be positive or negative
Only one of the following applies to a concave lens. This is : (a) focal length is positive (b) image distance can be positive or negative (c) height of image can be positive or negative (d) image distance is always negative
Since distance is always positive, speed is also always positive.