In the following passage some words have been deleted. Fill in the blanks with the help of the alternatives given. Select the most appropriate option for each blank.
Rivers around the world are (1) ______ with dangerous levels of antibiotics, (2) ______ to a recent study. Concentration of antibiotics in some waterways (3) ______ safety levels by 300 times. Researchers looked at several (4) ______ used antibiotics in rivers flowing (5)______ 72 countries.
Select the most appropriate option for blank No. 5