．The fact has worried many scientists ______ the earth is becoming warmer and warmer these years.

A．what

B．which

C．that

D．though

空气栓塞的发生原因不包括

A．输液时空气未排尽
B．输液完毕未及时拔除针头
C．加压输液、输血无人守护
D．输液管连接不紧密
E．连续输液添加液体不及时