问题
选择题
— 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
答案
答案：A
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。