The correct options that describe primary and secondary pollutants would be C and D.
Primary pollutants are substances released directly into the atmosphere capable of harming living organisms at a particular threshold concentration. An example is carbon monoxide. Secondary pollutants, on the other hand, are formed in the environment from primary pollutants. For example, NO2 is formed from the reaction of NO and oxygen.
<em>Secondary pollutants, therefore combine the toxicities of their component primary pollutants to arrive at stronger toxicity. Hence, secondary pollutants are more dangerous than their primary counterparts. </em>
