In wastewater treatment, some microbes are beneficial, some make no difference, and a few may be pathogens.

• A. Beneficial – pathogens
• B. Harmful – beneficial
• C. Beneficial – beneficial
• D. Pathogens – pathogens

Answer: (A)

In wastewater treatment, microbes can have different effects: some help break down waste (beneficial), some don’t affect the process (neutral), and a few can cause disease (pathogens). The best pairing reflects this mix by linking beneficial microbes with pathogens, since those are the two extreme roles highlighted. Beneficial microbes drive degradation and treatment processes, while pathogens pose public health risks that treatment aims to minimize. The other options mix up positive/negative roles or imply all microbes fall into a single category, which doesn’t fit the described spectrum.