Is this a z-statistic or t-statistic?

The water purifying company claims that the mean level of oxygen in the water that it processes is 5 mg. An environmentalist collects a liter of water from 30 different locations along the banks of a stream. He measures the amount of dissolved oxygen in each specimen. The mean oxygen level is 4.62 mg, with the standard deviation of 0.92. The environmentalist wants to know whether the oxygen level is less than 5mg. a. What are the null and research hypothesis? b. Is this a one tail or two tail analysis? one tailed c. What are the critical value(s). Assume a=0.05 for a two tail and 0.01 for a one tail test. a = 0, 01 Zev 2