A student was asked to write a stepwise procedure to demonstrate that carbon dioxide is necessary for photosynthesis. He wrote the following steps. The wrongly worded step is

(a) Both potted plants are kept in dark room for at least three days
(b) Bottom of the bell jars is sealed to make them air tight
(c) Both potted plants are kept in sunlight after the starch test
(d) A leaf from both the plants is taken to test the presence of starch.