When a converging thin lens forms a real image of an object placed on the left side of the lens, which statement best describes the sign of the image distance si under the standard sign convention?
a) si is negative because the image appears on the side of the lens opposite the object.
b) si is negative because real images formed by converging lenses are assigned negative distances.
c) si is positive because the image appears on the side of the lens opposite the object.
d) si is zero whenever the object is placed at the focal point.