The reason you can tell the difference between your mother’s voice and your voice or the difference between a clarinet and a flute is because of

   Timbre

Explanation: In addition to the fact that the mother's voice has a certain pitch and intensity, it also has its unique characteristic, which is a certain quality completely different, recognisable from the others, means unique. This uniquely recognisable characteristic of voice is called timbre, but also timbre is understood to mean sounds. Let's say every instrument has a distinctive sound, regardless of pitch or intensity. So, for example, we will distinguish the trumpet from the clarinet and together from the flute, although they play the same tone because they have a different timbre.

b) to encourage free trade

yes i would go with c

hope i

-lavira: )