It is well-known a person, especially the old, eats too much he’ll feel unfortable.
A. that
B. when if
C. if
D. that when
网友回答
A. that
B. when if
C. if
(答案→)D. that when
解析：考查：it做形式主语，真正地主语是后面的that 从句，that 从句中又是一个由when引导的时间状语从句。句意：众所周知。当一个人，尤其是老人，吃太多的时候他会感到不舒服的。故选D。