While main clauses, also known as independent, are groups of related words expressing a complete thought and therefore, can stand on their own, subordinate clauses do not have a full meaning on their own, and can not stand alone as full sentences. The sentence provided contains one main clause which it's “The Mason-Dixon line has been designated a National Historic Landmark,” and one subordinate clause: “which is perhaps the most famous border ever established by surveying methods,” whose function is to provide additional information about the noun “The Mason-Dixon line,” that is, to act as a non-defining relative clause.
C because a market research helps when you are buying something which he is (car) and this is a secondary resource because it is recommended by someone rather than them finding the research himself.
