Questions

The first indicator of an approaching thunderstorm is a buildup of dark clouds.

A thunderstorm is a storm with lightning and thunder; it is usually produced by heavy rain, gusty wind, and the cumulonimbus cloud. The thunderstorm is made up of moisture, unstable air and lift.

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.