阅读Passage 1，完成小题。
Today′s adults grew up in schools designed to sort us into the various segmentsof our social and eco-nomic system. The amount of time available to learn wasfixed : one year per grade. The amount learned by the end of that time wasfree to vary:some of us learned a great deal;some,very little. As we advancedthrough the grades,those who had learned a great deal in previous gradescontinued to build on those foundations.
Those who had failed to master the early prerequisites within the allotted timefailed to learn that which fol-lowed. After 12 or 13 years of cumulativetreatment of this kind ,we were ,in effect, spread along an achieve-mentcontinuum that was ultimately reflected in each student′s rank in class upongraduation.
From the very earliest grades, some students learned a great deal very quicklyand consistently scored high on assessments. The emotional effect of thiswas to help them to see themselves as capable learners, and so thesestudents became increasingly confident in school. That confidence gave them theinner emotional strength to take the risk of striving for more successbecause they believed that success was within their reach. Driven forwardby this optimism,these students continued to try hard, and that effort continuedto re-sult in success for them. They became the academic and emotional winners.Notice that the trigger for their e-motional strength and their learningsuccess was their perception of their success on formal and informalas-sessments.
But there were other students who didn′t fare so well. They scored very low ontests, beginning in theearliest grades. The emotional effect was to cause themto question their own capabilities as learners. They be-gan to lose confidence,which, in turn, deprived them of the emotional reserves needed to continue totake risks. As their motivation warned, of course, their performanceplummeted. These students embarked on what they believed to be anirreversible slide toward inevitable failure and lost hope. Once again, theemotional trigger for their decision not to try was their perception oftheir performance on assessments.
Consider the reality--indeed, the paradox--of the schools in which we werereared. If some students worked hard and learned a lot, that was apositive result, and they would finish high in the rank order. But if somestudents gave up in hopeless failure, that was an acceptable result, too,because they would occupy places very low in the rank order. Theirachievement results fed into the implicit mission of schools : the grea-ter thespread of achievement among students, the more it reinforced the rank order.This is why, if some students gave up and stopped trying(even dropped outof school), that was regarded as the student′s prob-lem, not the teacher′s orthe school′s.
Once again, please notice who is using test results to decide whether to strivefor excellence or give up in hopelessness. The \"data-based decisionmakers\" in this process are students themselves. Students are deci-dingwhether success is within or beyond reach, whether the learning is worth therequired effort, and so whether to try or not. The critical emotionsunderpinning the decision making process include anxiety, fear of failure,uncertainty, and unwillingness to take risks--all triggered by students′perceptions of their own capa-bilities as reflected in assessment results.
Some students responded to the demands of such environments by working hard andlearning a great deal. Others controlled their anxiety by giving up andnot caring. The result for them is exactly the opposite of the one societywants. Instead of leaving no child behind, these practices, in effect, drovedown the achieve-ment of at least as many students as they successfullyelevated. And the evidence suggests that the downside victims are morefrequently members of particular socioeconomic and ethnic minorities.
What has made students spread along an achievement continuum according to thepassage?

A. The allotted time to learn.

B.Social and economic system.

C. The early prerequisites students mastered.

D.Performance on formal and informal assessments.