Questions

The earth's atmosphere is composed of 4 layers. 

1. Troposphere

2. Stratosphere

3. Ionosphere

4. Mesosphere

The ozone layer is located in the lower region of the Stratosphere which extends at 11 - 50 kms from the Earth's suface.

Mercury

Looking at the diagram in  rows, the central circle equals half the sum of the numbers in the other circles to the left and right of the centre.

The receptor molecules, which receive signals from outside the cell, passing the signal to the inside.

In order for a nerve cell, or any cell, to perceive its environment, it needs a sensory apparatus. And for an individual cell, most of that sensation is derived at the cell surface. So, it expresses on its cell surface proteins that span across the membrane.

In the outside world, these proteins can act to receive signals, catching molecules on the outside and then as the molecule passes to the inside of the cell, they can send signals like alarm signals that can tell the cell that a particular molecule is outside.

Hence, here B is the receptor cell.