History: What is one of the most expressive features of additive synthesis?

What is one of the most expressive features of additive synthesis?

• These waves may change in volume, color, and space over time.
  At this level, we describe the physical properties.
• Additive synthesis is not expressive.
  This is a reductive answer.
• These waves stay static.
  A lack of change is boring and represents death.
• These waves may change in amplitude, frequency, and phase over time.
  That's correct!