Question

which chemical was used frequently as a pesticide until scientists realized that it persisted in the environment, biomagnifying up the food chain? radon phosphate roundup ddt

Explanation:

DDT (dichlorodiphenyltrichloroethane) was widely used as a pesticide. It was later discovered that DDT persists in the environment and biomagnifies up the food chain. Radon is a radioactive gas, not a pesticide. Phosphate is often used in fertilizers, not as a pesticide with the described environmental issue. Roundup (glyphosate - based herbicide) has different environmental concerns compared to the biomagnification and persistence described for DDT.

Answer:

DDT