GWD-10-Q2：which指代

Teeth123

Marconi’sconception of the radio was as a substitute for the telephone, a tool forprivate conversation; instead, it is precisely theopposite, a tool for communicating with a large, public audience.

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

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

C.   Marconi conceived of the radio as a tool forprivate conversation that could substitute for the telephone; instead, it hasbecome

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

       E .Marconi conceived of the radio to be a substitute forthe telephone, a tool for private conversation, other than what it is
我现在已经完全搞晕了，
为什么A和C的it可以指代the radio，是因为it的性质还是因为那个分号呢？
B 中的which在but后面又做主语，不可以指代前一句的主语吗？那么真正which指代的应该是the telephone还是conversation 呢？
谢谢！

supershuang

A的it指conception