【单选题】

People faint (晕倒) when the normal blood supply to the brain is suddenly out down. This can happen if they are surprised or shocked by sudden news or something they see. Some people faint if they see others hurt. Some people faint in crowds. Others faint if they are a in a room that is hot and stuffy (不通气的，闷热的).

If a person faints while standing, lay him down. If his face is pale, lift his feet. If he is sitting down when he faints, place his head between his knees. Loosen any tight clothing that might keep him from breathing easily. If possible, place a cold, wet cloth on his forehead (前额).
If you feel faint when sitting, put your head______.A．against the chair back
B．between your knees
C．on your chest
D．to one side