Statistical Sampling

Parts (a) to (d) are based on the following information:

A study on wellbeing and stress of university students is conducted. The researcher drew a random sample of 5 departments on campus. Then he randomly sampled 100 students within each department that was selected and handed them the link to his online survey. Ultimately, only 30% of the sample responded. Amongst those who responded, 120 of them say they feel stressed.

(a) What sampling method was used?
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