21. (D)
Question Type: Synthesis (20-22, 58-69)
Looking back at the line reference given to us in the question, we see that the generalization is that caries tends to increase as reliance on agriculture increases. The final paragraph of Passage B tells us that the Ban Chiang population does not fall in line with the generalization made in passage A, thus making (D) the correct answer.
(A) is contradicted. Furthermore, no single example could confirm a generalization of this nature"”"in general, the greater..""”so cross out (A) twice!
(B) is contradicted.
(C) is wrong because a non-conforming example is relevant.
(E) is too extreme. A single example can’t disprove a generalization of this nature.