答案：C/E.
翻译：这是那所上周有一些德国朋友参观的学校吗?
此题最好还原成陈述句来判断答案.
1.答案A还原成：This school is where some German friends visited last week.
即where some German friends visited last week做表语从句,但是该表语从句中的谓语动词visited明显缺少宾语what,而不缺少地点状语where.
由此可以排除A.
2.答案B还原成：This school is one some German friends visited last week.
即用定语从句some German friends visited last week修饰主句的表语（即先行词）one,但是one为泛指的school,而不能代指后被定语从句所修饰的特定成分this school,如果是the one才可以.
由此可以排除B.
3.答案D还原成：This school is some German friends visited last week.
很明显,some German friends visited last week不能单独做此句的表语从句,因为此从句中的谓语动词visited后面缺少一个宾语,必须选用连接代词what来代替并引导该从句.
由此可以排除D.
4.答案E还原成：This is the school that some German friends visited last week.
主句为This is the school.定语从句that some German friends visited last week修饰主句的表语（即先行词）the school.关系代词that代替先行词the school,在该从句中充当谓语动词visited的宾语,而且可以省略that.
答案C还原成：This is the school some German friends visited last week,即是答案E中省略that的句子.
所以,该题正确答案为C/E.