试题题干
Fill in the blanks with an item as required.
With a relative pronoun:
The microscope is a very useful instrument without 
scientists are unable to see things too small to be seen with the naked eye.
参考答案
试题解析
本句考查关系代词，从句先行词是microscope，需要指代的是名词且先行词离的比较远的时候应该用which。
句意：显微镜是一种非常有用的仪器，没有它，科学家们就看不清太小而无法用肉眼看到的东西。