Questions: Eighty percent of antibiotics used in the United States are: injected in the drinking water supply. for livestock. for children. for the elderly.

The answer is the second one: for livestock.

Explanation for each option:

1. Injected in the drinking water supply: This option is incorrect. Antibiotics are not typically injected into the drinking water supply for human consumption. This would pose significant health risks and is not a standard practice.

2. For livestock: This option is correct. A significant portion of antibiotics used in the United States is administered to livestock. This is done to promote growth and prevent disease in animals raised for food production.

3. For children: This option is incorrect. While antibiotics are prescribed to children for various infections, they do not account for the majority of antibiotic use in the United States.

4. For the elderly: This option is incorrect. Although the elderly may require antibiotics for certain medical conditions, they do not represent the largest group for antibiotic use in the U.S.

In summary, the majority of antibiotics in the United States are used for livestock, making the second option the correct answer.