Which Best Describes Platypnea

Which best describes platypnea?

A) Onset of dyspnea when the patient assumes the sitting position from the supine position
B) Onset of dyspnea when the patient assumes the prone position from the supine position
C) Onset of dyspnea when the patient assumes the sitting position from the standing position
D) Onset of dyspnea when the patient assumes the standing position from the prone position