Hypothesis Testing using a single sample

I need help with this:

Water samples are taken from water used for cooling as it is being discharged from a power plant into a river. It has been determined that as long as the mean temperature of the discharged water is at most 150 Degrees F, there will be no negative effects on the river ecosystem. To investigate whether the plant is in compliance with regulations that prohibit a mean discharge water temperature above 150 Degrees F, a scientist will take 50 water samples at randomly selected times and will record the water temperature of each sample. She will then us a z statistic to decide between the hypotheses Ho: u=150 and Ha: u > 150, where u is the true mean temperature of discharged water.

This is what I have to do.

A) Explain why use of the z statistic is appropriate in this setting.

Plz Help.

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