Final answer:
The threshold limit set by the law determines the validity of John's prosecution if residual traces of the drug were found in his system.
Step-by-step explanation:
Under the law, the prosecution against John would likely be invalid if the residual amount of the drug found in his system on July 7th was below the threshold limit set by the law. The law may establish a specific limit for the drug in question, below which the presence of the drug is not considered illegal. This threshold limit is determined based on factors such as the pharmacokinetics and metabolism of the drug, as well as the potential for trace amounts to remain in the body even after discontinuing use.