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

Doesnt the use of instead make the second clause dependent on the first with the use of semicolon??