prep SC 一道题不会做，求助！

azhongzhong

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

这道题我选的E，答案是C，CE不会比，求牛人指点，谢谢！

nobody910

为什么我看的答案是E。。。虽然我选的C   LZ在何处看到的答案？