akt7151) Normally, "comma + -ing" modifiers are adverbial. These modifiers often show the RESULT of an action. So in this case, the sentence is saying that because the tunnel was completed, these two places were now linked. The correct answer, C, adds an additional -ing modifier to extend the result: the completion of the tunnel also permitted cars to pass through.
2) Normally, "which" modifiers apply to the preceding noun. This is one problem with A: we don't want to modify "higways," but as I've shown above, it's not necessary to modify a noun at all. If you're feeling confused about the correct application of this modifier in A, well, that's why we don't like it! A noun modifier should clearly apply to one noun, generally the noun right next to it. Adverbial modifiers have a bit more freedom, since they apply to entire clauses. This allows them to appear before, after, or even in the middle of a clause.