Questions

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.

Mercury

Parabola, because when the coin is dropped at a particular point, it has the same velocity as the running train. So, by Newton's first law, it continues its motion horizontally with that velocity. But then, it is also pulled down by earth's gravity continuously. In this situation its horizontal motion gives way to the parabolic motion until it reaches the ground.

What has Hands But cannot Clap is a Clock or a Watch.

A Clock technically has 3 hands.

1 Hour Hand Which tells Hours

1 Minute Hand Which tells Minutes

1 Second Hand Which tells Seconds.

But cannot clap with these hands.