When bacteria are used in wastewater treatment plants to convert hydrogen sulfide into hydrogen sulfate, why are the bacteria grown on biofilms?

A) To protect them from damage to their cell walls caused by the waste water.
B) The bacteria that make this conversion can only grow in biofilms.
C) The biofilms increase the time during which the bacteria can metabolize the hydrogen sulfide.
D) The hydrogen sulfate adheres to the biofilm, reducing the amount of this chemical released in treated water.
E) Only bacteria growing in biofilms produce the enzyme necessary to make this conversion.