It has been found that boys' English grades are up to a tent

选A.when
It has been found that boy's English grades are up to a tenth worse 【when】 there are much more girls in the class with them,but girls' grades are unaffected.
研究发现,当班上女孩人数更多的时候,男孩的英语成绩就下降10%,但女孩的成绩则不受影响.
句子想表达学生的成绩在何时或何种情况下会发生变化,所以用when.
其他的词不符合上下文.

C .that .

【选择】A. when

【解析】本题是个多重复合句，其语法结构如下：

• 形式主语：It

• 被动式的复合谓语动词hasbeen found 《（主句所说的那个事实）已经得到证实》

• 主语从句：that boys' English grades are up to a tenth worse when there are much more girls in the class with them, but girls' grades are unaffected 其中——

　　【嵌套并列主句】boys' English grades are up to a tenth worse, but girls' grades are unaffected 《男生的英语成绩多达十分之一更加糟糕，女生的成绩则不受到影响》

　　【嵌套时间状语从句】when there are much more girls in the class with them 《如果班里有比男生更多的女生掺杂进来的时候，（那么男生的英语成绩就...）》