precisely the opposite, a tool for communicating with a large, public audience.

A Marconi's conception of the radio was as a substitute for the telephone, a tool for private conversation; instead, it is 

B Marconi conceived of the radio as a substitute for the telephone, a tool for private conversation, but which is  

C Marconi conceived of the radio as a tool for private conversation that could substitute for the telephone; instead, it has become  

D Marconi conceived of the radio to be a tool for private conversation, a substitute for the telephone, which has become  

E Marconi conceived of the radio to be a substitute for the telephone, a tool for private conversation, other than what it is, 

当我们无法一眼看出原句的毛病是，可以从选项的区别入手。这里A选项的主语相比于另外四个不同，中心词落在了conception上，其它四个是Marconi本人。这时我们需要考察的局部句意关系，可以从主谓入手。主语和谓语的一致，属于动作发起者和动作之间的局部句意关系。

因为A选项的谓语项是was a substitute for thetelephone(*GMAT中我们姑且把主系表看作主谓宾的一种。——可达注)，这显然不能用来描述conception。所以干掉。