According to the passage, the age range of some businesses' customers may be limited because of ______.

A．Airborne dust may trigger suppressor macrophage suicides.
B．Airborne dust may trigger stimulatory macrophage suicides.
C．The ratio of suppressor macrophages to stimulatory ones in people with lung diseases is much higher than that in people with healthy lungs.
D．Only stimulatory macrophages show responses to airborne dust.

A．is the only agency studying the effects of air pollution on health
B．has launched a surprised attack on researchers
C．has decided to decrease the permitted concentration of airborne dust
D．has tremendously improved pollution control