— Why are you so popular with your students?
— As a teacher, I seldom give my students so difficult a problem they cannot work out.
A. as
B. that
C. if
D. which
网友回答
(答案→)A. as
B. that
C. if
D. which
解析：考查定语从句。先行词为a problem，后面的从句they cannot work out后缺少宾语，所以使用关系代词，so… as，为固定搭配，所以选A。