Argument suggests: people who stayed in teaching profession earn on average much less than do people who moved out of teaching and pursured some other career.
, the correct answer, says that both groups are comparable in terms of factors that determine their pays. If the group of people, who moved out of teaching profession, picked up some nische skills (such as programming, management etc) or higher degrees (Masters, PhDs) then these two groups are not comparable in skills or in their earnings. later group are paid more for better education or for more demanding skills. Therefore, argument and the statistics hold good as long as these two group of people have same skills, educational qualifications etc.
"Education is what remains when one has forgotten everything he learned in school."