Which situation is an example of opposite quantities combining to make 0?

A gift card had a balance of $20. Then, all $20 on the gift card was spent.

B A swimmer was 6 feet below the surface of the water. Then she descended another 6 feet.

C A container had 2.1 ounces of liquid. Then, 2.1 ounces of liquid was added to the container.

D The temperature was -5 degrees Fahrenheit. Then, the temperature decreased by 5 degrees

Stu and ambrose are working together on a study of whether diet is related to students' mental health. first, they randomly select 10 students from the student center and 10 students from the office of counseling and psychological services. then, they ask students to rate their diet in terms of healthiness on a scale of 1-100 before completing a 20-item mental health survey. a) name one of the constructs being studied b) how was the construct you named above operationally defined in this experiment? e) was the variable you described above their independent variable, their dependent variable, or a controlled variable? d) what is a confounding variable? what is a potential confound in this study?