A concave mirror is placed on a horizontal table, with its axis directed vertically upwards. Let O be the pole of the mirror and C be its centre of curvature. A point object is placed at C, whose real image is also formed at C. If the mirror is now filled with water, the image will be
(a) real, and will remain at C
(b) real, and will be located above C
(c) virtual, and will be located below O
(d) real, and will be located between C and O