prep2-72 请教

72.       (28395-!-item-!-188;#058&003974)

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 having improved 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

想不明白E为什么不用have呢？难道是 "mastery"做单数处理？请教！