Final answer:
The correct answer is B) 98%, which indicates that storm sewer runoff is responsible for a very high percentage of pollution resulting in beach advisories for swimmers in the US.
Step-by-step explanation:
The majority of beach advisories and closings in the US are due to high levels of pollution often caused by storm sewer runoff. Storm sewer runoff can contain contaminants from various non-point sources like streets, lawns, and agricultural lands that get washed into the water bodies during heavy rainfalls. Monitoring agencies measure the levels of pollutants in the water and issue advisories if they exceed safe limits for swimming and other recreational activities. When considering the options given A) 5 B) 98 C) 50 D) 21, option B) 98% is the correct answer, signifying that a very high percentage of pollution leading to beach advisories is attributed to storm sewer runoff.