:2.   After a Sociology test, each student leaves his (or her)
test on top of the desk at which he (or she) has been sitting. 
After all the students have left, the instructor tries to get an
idea of how the class as a whole has done on the test by first
grading the exams from the desks in the front row.  The sample of
exams he has selected to grade first is best described as a <p>

a.  simple random sample <p>

b.  sample of convenience <p>

c.  half-sample <p>

d.  stratified sample <p>

e.  two-stage sample <p>