请教PREP2 133

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A recently published report indicates that the salaries of teachers continue to lag far behind other college-educated professionals, because they make an average of nearly $8,000 a year less at the start of their careers and almost $24,000 less a year by the time they reach the age of 50.

(A) other college-educated professionals, because they make an average of nearly $8,000 a year less at the start of their careers and almost $24,000 less

(B) other college-educated professionals, by an average of nearly $8,000 a year at the start of their careers, to almost $24,000

(C) what other college-educated professionals are paid--making an average of nearly $8,000 a year less at the start of their careers and almost $24,000 less

(D) those of other college-educated professionals--by an average of nearly $8,000 a year at the start of their careers to almost $24,000 less

(E) those of other college-educated professionals--by an average of nearly $8,000 a year at the start of their careers, and by almost $24,000

正确答案是E，可是我想问一下解释里面说less和lag behind是重复的，怎么解释啊？谢谢大家。

salaries are lag ,by $8000, behind those of other proffesionals

salaries are 8000 less than those of other proffesionals

see the problem?