Answer: C. 20 containers
Step-by-step explanation: There are 1000-mL in 1-L. Since you are filling a 5-L fish tank, you must multiply 1000 by 5 to find the total number of mL in the 5-L tank, 5000 mL. Now, you have to divide 5000 by 250 to find the total number of full containers required to fill the fish tank. Therefore, your answer would be 20 containers.