Question

a group of students is trying to determine which detergent will best remove an oil stain. they stain five separate cloths with oil and then wash each of the cloths in a different detergent. they repeat each test two more times. which step should be included in their investigative plan? use a different type of oil for each test. vary the size of the oil stain on each piece of cloth. change the amount of detergent used during each trial. place each oil stain on separate, but identical, pieces of cloth.

Explanation:

In a scientific experiment to test the effectiveness of detergents on oil - stains, variables like type of oil, size of stain, and amount of detergent should be controlled. Using identical pieces of cloth helps control the fabric variable, ensuring that any differences in stain removal are due to the detergents and not the cloth.

Answer:

D. Place each oil stain on separate, but identical, pieces of cloth.