Based on the results of the tests performed in Exercises 1–3, which of the following is the best conclusion?

i. We can conclude that the median number of species increases as the solids retention time increases from 3 to 12 and from 12 to 30 days.

ii. We cannot conclude that the median number of species increases as the solids retention time increases.

iii. We can conclude that the median number of species increases as the solids retention time increases from 3 to 12 days, but it may not continue to increase as the retention time increases to 30 days.

iv. We can conclude that the median number of species increases as the solid retention time increases from 12 to 30 days, but it may not increase as the retention time increases from 3 to 12 days.