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.
【正确答案】：which
【题目解析】：先行词是物体，关系从句可以用which或者that引导，但是如果做介词宾语，而且介词和引导词一起在关系从句的句首，就只能用which。
