答案选 C
Only when you go through the mental exercises to come up with a tentative definition should you open the dictionary to see if you’re right
只有当你通过练习在心里掌握了一个初步的定义,你才应该打开字典,看看你是不是对的.
这里考查了一个倒装句（从should you可看出）,还原成正常顺序的句子成分：
when you go through the mental exercises to come up with a tentative definition 条件状语从句
you should open the dictionary to see if you’re right 主句