Question

How do musical instruments vary pitch

Answer 1

by increasing the amplitude of standing waves

by changing the frequency of standing waves

by interrupting sound waves

by changing the wave length of standing waves

Answer 2

Higher notes have tighter strings, lower notes have looser strings.

With air operated instruments the lower the space for air to go through the higher the pitch. That's why a tuba has such a low note- because it is very large and wide.