is well known to the Irish is St. Patrick was taken to Ireland as a prisoner when he was

网友回答

A. It, that
B. As, what
C. That, what
(答案→)D. What, that
解析：原句意思是，爱尔兰人都知道，帕特里克在他十六岁时，作为囚犯，被带到了爱尔兰。这是个主从复合句，包含一个主语从句和一个表语从句。主语从句中缺主语，故用what引导；后面的表语从句中不缺成分，故用that引导，所以选D。