GMAT Sentence Correction (SC) | The first shots of the American Revolution, fired at

I need explanation with regards to the following:

In option A it says ...... ", but news of these shots, which were sent by swift messenger " wouldn't"which " here, separated by the comma modify the entire phrase before it, hence modifying the news of these shots? In what cases"which " modifies the entire phrase preceding it and in what case it modifies the noun preceding it?

In option B and C what is"that " modifying...theshots?



"The first shots of the American Revolution