How many years would it take for Earth to recover to pre-1950 ozone levels if all of the ozone-depleting substances were banned tomorrow? 

A)  about 50 years 
B)  about 75 years 
C)  about 100 years 
D)  about 125 years

It would take about 100 years for Earth to recover to pre-1950 ozone levels if all of the ozone-depleting substances were banned tomorrow. This is because some of these substances, such as chlorofluorocarbons (CFCs), have a long atmospheric lifetime and can remain in the atmosphere for up to several decades. Therefore, even if all production and use of these substances were to cease immediately, it would still take a significant amount of time for ozone levels to return to pre-1950s levels.