What is the sound of one hand clapping?

I take attendance by having the students answer a question.

What is the sound of one hand clapping?

• very quiet
• waft of air
• om
• a friendly wave
• Basically that of 2 but quieter.
• whoosh?
• flap
• whoo
• blob

This post's theme word is crural, "relating to the leg." The crural version of this puzzle is much easier; one foot stamping is coherent on its own.