请教FeiFei-20

本人很笨，可能很多人觉得很简单，但我就是不懂

20. A rise in the percentage of all 18-year-olds who were recruited by the armed services of a small republic between 1980 and 1986 correlates with a rise in the percentage of young people who dropped out of high school in that republic. Since 18-year-olds in the republic are generally either high school graduates or high school dropouts, the correlation leads to the conclusion that the republic’s recruitment rates for 18-year-olds depend substantially on recruitment rates for high school dropouts.

Which one of the following statements, if true, most weakens the argument?

A.        A large number of 18-year-old high school graduates were recruited for the republic’s armed services in 1986 than in 1980.
B.        Many of the high-technology systems used by the republic’s armed services can be operated only by individuals who have completed a high school education.
C.        Between 1980 and 1986 the percentage of high school graduates among 18-year-olds recruited in the republic rose sharply.
D.        Personnel of the republic’s armed services are strongly encouraged to finish their high school education.
E.        The proportion of recruits who had completed at least two years of college education was greater in 1986 than in 1980.

答案C，我想不通C是怎样削弱原文结论的，请哪位好心人说说这怎么推的，尽量详细，我理解能力不强，谢谢！