问题补充：
On the table she found a piece of paper some confusing symbols.
A. which was written
B. that was written
C. on which were written
D. on that was written
答案：
A. which was written
B. that was written
(答案→)C. on which were written
D. on that was written
解析：从句可写成：some confusing symbols were written on the paper, 故用on which。
