The big fact to rely on here is that a majority of people make a profit. The issue to know is that you can infer that there is some overlap with any group which we learn also is more than half. For example, if I know there are 10 people in a room and most are men and most are under 6 feet tall, I can infer that there is at least one man who is under 6 feet tall. However, if I only know that of those 10 people that some are lefties and that some are elderly, I cannot say that there is an elderly lefty. Perhaps there are 3 lefties and 3 elderly individuals. Finally, we cannot say that there is a man who is a lefty. Perhaps there are 6 men in the room (most) and 3 lefties (some), and no overlap between the two groups.
So, back to the problem, with (C) we learn that some people do research but rely on their own info, but perhaps that's a small number that does not overlap with the majority who make a profit.
With (D), we don't know the number of folks who do not do research -- we only learn that it is "some". Perhaps it's a small number and, again, perhaps it doesn't overlap with the majority who make a profit.
Does that make sense?