P. 60 Q9

I am struggling with these main point questions. Usually I bracket the thesis correctly, and I have no trouble eliminating 3 of the 5 choices usually, but often find aspects that look attractive in the remaining 2 choices. I bracketed the "But when Marshall..." in line 6-10 as the thesis, and also bracketed the "but the techniques he honed..." in 58-63. So, I narrowed it down to B and D on this one. At first I crossed out B because I couldn't find that Marshall devised the strategy "in pursuit of the NAACP's civil rights goals." How do we know that was his reason for devising the tactics? And really this is the only reason I didn't choose B. I chose D because it included the originally considered a radical departure bit, and because the tactics became the norm. I am still unsure about B, but I think I figured out why D is wrong. Is it because he didn't really adapt them in such a way that they became accepted, but maybe the firms adapted them instead. And also, "the field of law" is too general, and we only know about pubic interest litigation??? What if it said "a field of law"? I'm curious about how particular the LSAT is with definite/indefinite articles. I'm also interested in hearing your feedback on that first part of B, and also why D is wrong.

Thanks!