Photochemical Smog Forms When Exhaust from Vehicles Reacts with Air

Photochemical smog forms when exhaust from vehicles reacts with air in the presence of sunlight to produce an ozone-rich brown haze. Without government regulation,

A) the frequency and occurrence of photochemical smog may decrease, leading to improved atmospheric quality.
B) photochemical smog is not an issue now, therefore should not be regulated.
C) photochemical smog would no longer be an issue.
D) the frequency and occurrence of photochemical smog may increase, leading to poor atmospheric quality.