Please help with a question with no solution

The registrar of a school wants to estimate how many students are living at home. Determine whether the following schemes of sampling students are simple random sampling, stratified sampling or neither. Explain your answers.

(a) The registrar keeps an alphabetical list of students. It is proposed that a number be chosen at random from 1 to 100 and the name that far down in the list and every hundredth name after it be selected.

(b) It is proposed that each school in the university chooses a predetermined number of randomly chosen students

(c) It is proposed that someone should stand in front of the library and select a random sample of entering students.

Thank you!