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:

1. Both potted plants are kept in dark rooms for at least three days.
2. Bottom of the bell jars is sealed to make them air tight.
3. Both potted plants are kept in sunlight after the starch test.
4. A leaf from both the plants is taken to test the presence of starch.

If a lens can converge the sun rays at a point 20 cm away from its optical centre, the power of this lens is –

1. +2D
2. –2D
3. +5D
4. –5D

The radius of curvature of a converging mirror is 30 cm. At what distance from the mirror should an object be placed so as to obtain a virtual image?

1. Infinity
2. 30 cm
3. Between 15 cm and 30 cm
4. Between 0 cm and 15 cm

Respiratory structures of two different animals — a fish and a human being are as shown.

Observe (a) and (b) and select one characteristic that holds true for both of them.

1. Both are placed internally in the body of animal.
2. Both have thin and moist surface for gaseous exchange.
3. Both are poorly supplied with blood vessels to conserve energy.
4. In both the blood returns to the heart after being oxygenated.