John attached a ball to a spring

Trumpeter a holds a b-flat note on the trumpet for a long time. person c is running towards the trumpeter at a constant velocity. person b is running away from the trumpeter at the same rate. person d is standing still the whole time. which person hears a frequency that is lower than the b-flat?

Why there is no inductive reactance in dc circuits

Which changes of state do the labels represent? a: b: c: