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.

Comprehension:
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.