What causes the pre-selected volume of air to flow into the patient by communicating directly with a breathing circuit?

What causes the pre-selected volume of air to flow into the patient by

1.	What causes the pre-selected volume of air to flow into the patient by communicating directly with a breathing circuit?
A.	Nebulizer
B.	Microcontroller
C.	Bellows
D.	Humidifier
Answer» D. Humidifier