第21题21. Historian: We can learn about the medical history of individuals through chemical analysis of their hair. It is likely, for example, that Isaac Newton’s psychological problems were due to mercury poisoning; traces of mercury were found in his hair. Analysis is now being done on a lock of Beethoven’s hair. Although no convincing argument has shown that Beethoven ever had a venereal disease, some people hypothesize that venereal disease caused his deafness. Since mercury was commonly ingested in Beethoven’s time to treat venereal disease, if researchers find a trace of mercury in his hair, we can conclude that this hypothesis is correct. Which one of the following is an assumption on which the historian’s argument depends? (A) None of the mercury introduced into the body can be eliminated. (B) Some people in Beethoven’s time did not ingest mercury. (C) Mercury is an effective treatment for venereal disease. (D) Mercury poisoning can cause deafness in people with venereal disease. (E) Beethoven suffered from psychological problems of the same severity as Newton’s. 答案为B，如何解释？ | 
