Help with a,b,cThanks

Problem #3: In a study conducted to determine the preference of students regarding the teaching modality for summer quarter, a random sample of students who registered for classes as of May 2022 was selected for interview. The students were split according to class level (Freshman, Sophomore, Junior, Senior, Graduate), and from each level, a random sample of 100 students were selected. a. What sampling technique was applied to generate the sample of students? b. What type of variable teaching modality (online, blended, face-to-face) is, and what is its level of measurement? c. What type of bias is expected by just sampling from students who registered as of May 2022? Explain your