In one of the earliest Data Sufficiency posts on this blog, which you can read here and here, we discussed how the GMAT test-makers use the C-Trap to lure test-takers into making a mistake. The beauty of such questions is that the test-takers d not even realise that they have made a mistake. On the contrary they are very confident that they have answered it correctly. The C-Trap is set to lull test-takers into thinking that they can easily get the answer by using both the statements. But while both statements together will give you the answer, the question you need to ask is whether both statements are in fact required. Remember you need to choose option (C) only if both statements ARE required.