When I try to find ______that prevent so many people from taking part in the program, it seems to me that there are two main causes.

A．why it does

B．what it does

C．what it is

D．why it is

经口气管插管机械通气的病人突然出现全身发绀，SpO2下降至30～40%，下列哪个原因可能性最大?

A、血氧饱和度监护仪故障 

B、呼吸机通气参数设置不当 

C、气管插管脱出 

D、气道痉挛