Two machines are used to fill cans. The machines are supposed to fill each can with 12 ounces of liquid. The amounts actually filled follow a normal distribution. A simple random sample of 10 cans is filled by each machine. The question of interest is whether the variability in the fill volume differs between the two machines. To address this question, which of the following is the most appropriate type of hypothesis test?

i. A test for the difference between two population means using independent samples ii. A test for the difference between two population proportions iii. A test for the difference between two population means using matched pairs iv. A test for the difference between two population standard deviations