John was drug-free for almost six months. then he started hanging out with his addict friends, and he has now started abusing drugs again. this is an example of

Relapse is a medical term used to refer to a recurrence of a past condition. In this case is a restored "drug-seeking behavior", what is fairly common in individuals who have developed a drug addiction. In this scenario it occurred due to the environment influences (his addict friends).

b) to encourage free trade

explanation:

allergens: overstimulate the nerve system

neurotics: attack nerve cells

mutagens: damage or otherwise alter dna sequence

generators: cause defects during growth and development  

carcinogens: cause unchecked cell growth

i hope you get the idea according to me these are the effects that happened.