V. Choose the letter A, B, C or D to complete the passage below. Water pollution happens (21) __________ toxic substances ent

V. Choose the letter A, B, C or D to complete the passage below.
Water pollution happens (21) __________ toxic substances enter water bodies such as lakes, rivers, oceans and so on, getting dissolved in them, lying suspended in the water or depositing on the bed. This degrades the quality of water. Not (22) __________ does this spell disaster for aquatic ecosystems, the pollutants also seep through and reach the groundwater, which might end up in our households as contaminated water we use in our daily activities, including drinking.
Water pollution can be (23) __________ in a number of ways, one of the most polluting being city sewage and industrial waste discharge. Indirect sources (24)__________ water pollution include contaminants that enter the water supply from soils or groundwater systems and from the atmosphere via rain. Soils and groundwater contain (25)__________ residue of human agricultural practices and also improperly disposed of industrial wastes.
Pollutants can be of varying kinds: organic, inorganic, radioactive and so on. In fact, the list of possible water contaminants is just too vast to be listed here.
21. A. what B. when C. why D. while
22. A. both B. only C. well D. either
23. A. cause B. caused C. causing D. causes
24. A. of B. in C. at D. for
25. A. a B. an C. the D. x