(C) It was so secret that a recording system was installed and operated in the Kennedy Oval Office
In option C, reference of pronoun it is not clear.
It is not a place holder. it is interrupted by idiom so... that therefore that clause can not replace it.
If I say it= Plan
Plan was so secret that a recording system was installed and operated in the Kennedy Oval Office (Plan make sense)
But we can not infer exactly what should be the antecedent of It.
In option C, reference of pronoun it is not clear.
It is not a place holder. it is interrupted by idiom so... that therefore that clause can not replace it.
If I say it= Plan
Plan was so secret that a recording system was installed and operated in the Kennedy Oval Office (Plan make sense)
But we can not infer exactly what should be the antecedent of It.
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