Which of the following is the most serious effect of water pollution for humans?
a. infectious diseases
b. algal blooms
c. toxic food chain effects
d. low-oxygen water select the best answer from the choices provided a b c d

Of course, infectious disease seems like a terrible thing to be added to a water supply, and of course it is, but in reality this only occurs rarely in modernised societies. Diseases can spread rapidly from contaminated drinking supplies - water is the most vital sustaining chemical to human life after oxygen - and must be considered the most serious effect of water pollution.

I think it's B, it just makes the most logical sense to me.