The correct option is C dissolved heavy metals
The cleaning of the Arcata marsh occured in two stages. First the conventional sedimentation, filtering and chlorine treatments were given. After this stage, lots of dangerous pollutants like dissolved heavy metals still remained. To combat this, the biologists developed a series of six connected marshes over 60 hectares of marshland. Appropriate plants, algae, fungi and bacteria were seeded into this area, which neutralised, absorbed and assimilated the pollutants. Hence, as the water flowed through the marshes, it got purified naturally.