求教 PREP part2 第72题

Several educational research groups are denouncing the mayor's proposal of a system by which individual teachers would be given raises or bonuses should it be that test scores show their students' mastery of reading skills havingimproved over a nine-month period.

(A) should it be that test scores show their students' mastery of reading skills having
(B) should it be that test scores show their students' mastery of reading skills have
(C) should test scores showing their students' mastery of reading skills has
(D) if test scores showing their students' mastery of reading skills having
(E) if test scores show that their students' mastery of reading skills has

答案E。我也选的E，但是有不明白的地方就是解释中说E是一个条件状语从句，但我之前在很多题看到的规则是 “如果条件状语从句用一般现在时，那么主句用一般将来时” 。如果用E带入的话，为什么在这里(相对)主句动词是 "would be given"，而不是 “will be given” ？我之前有看到过题就是区别will和would的，理由是条件状语从句中用的是一般现在时，所以选will 排出would。
还请指教！谢谢