To find how high school students feel about hot lunches, adam walks to the nearest high school and gives a survey postcard to every twentieth student that enters the building. he asks the students to mail the postcard in if they choose to participate in the study. select all sampling methods used. cluster convenience simple random stratified systematic voluntary

The surveyor selects the twentieth student who enters the building. The student do not enter the building to be surveyed but for other reasons and the researcher take advantage of that and selects them. This is a form of continence sampling.
 The researcher selects every twentieth student and not any other. Therefore, the sampling is systematic in nature.
Finally, students selected to participate in the survey are free to choose if to participate in he survey or not. Therefore, it is voluntary kind of sampling.

Therefore, the applicable sampling methods are convenience, systematic, and voluntary.