读者学会这些基本概念很重要，否则后面内容的讨论理解起来会有困难。It is important for the reader to learn the basic concepts well，（）the discussion of later topics will be difficult to understand.
读者学会这些基本概念很重要，否则后面内容的讨论理解起来会有困难。It is important for the reader to learn the basic concepts well，（）the discussion of later topics will be difficult to understand.
A.or
B.but
C.and
D.for
正确答案：or