13. Of those person who became teachers in 1968 and who later left the profession, 30 percent today earn salaries above $35,000 a year: of those who became teachers in 1968 and have remained in the profession, only 15 percent today earn salaries above $35,000 a year. These figures indicate how underpaid teachers are today.

The argument above depends on which of the following assumptions about the persons for whom statistics are cited?

(A) At least one-third of the group of persons who have remained in teaching would today be earning more than $35,000 a year if they had left teaching.

(B) The group of persons who left teaching and the group who did not are comparable in terms of factors that determine how much people outside the teaching profession are paid.

(C) Most of those persons who left teaching did so entirely because of the low salaries teachers earn.

(D) As a group, those persons who have remained in teaching are abler and more dedicated than the group of persons who left teaching.（B）

(E) The group of persons who left teaching and who today earn more than $35,000 a year were more capable teachers than the group who remained in the profession.

感觉(B)不是很严密；把(B)取非，factors are not comparable, 但是如果离开学校的老师能力比没有离开的能力强，则对原文是加强，而不是削弱。我觉得比较严密的假设应该是"离开学校的老师能力并不比没有离开的能力强"，而不是“两者的能力相当.

I think your understanding is not entirely correct.

"comparable in terms of factors that determine"

Here the factors not only refers to 老师能力 but also other factors such as  companies, environment, etc.

So B says only when you assume that all these factors that determine how these people get paid are the same, then you can conclude that teachers are under paid.