"The aroma of wine is not afraid of the depth of the alley" because the molecules contained in alcohol are constantly moving and spreading around, allowing people to smell the aroma of wine.

A. From the properties of molecules, it can be seen that the main reason for smelling the aroma of wine has nothing to do with the composition of the molecules, so the option is wrong.

B. Since molecules are constantly moving, this is the main reason for the smell of wine, so the option is correct.

C. From the properties of the molecules, it can be seen that the main reason for smelling the aroma of wine has nothing to do with the space between molecules, so the option is wrong.

D. From the properties of molecules, it can be seen that the main reason for smelling the aroma of wine has nothing to do with atoms being the smallest particles in chemical changes, so the option is wrong.

So choose B.