Why might someone want to have smoke in the air at a laser show

Someone wants to have smoke in the air at a laser show because It's the dust particles or mist droplets which reflect the light of the laser. When you see a laser beam, what you are seeing is the beam being scattered by various small particles suspended in the air. ... When people want to show off laser beams, they often add smoke or equivalent to the air.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.