The Most Significant Contribution of Studies Like the High/Scope 20-Year

The most significant contribution of studies like the High/Scope 20-year study of preschool children is:

A) the evidence that boys consistently have higher academic achievement levels than girls
B) the finding that the high cost of exemplary pre-school programs outweighs the total social benefits of such efforts
C) the indication that the quality of teaching is the most important factor in student achievement
D) their documentation of the benefits of a good pre-school program for disadvantaged children