worries the doctor most was they could find the cause of the disease.A. What; howB. That

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(答案→)A. What; how
B. That; that
C. What; what
D. How; what
解析：考查名词性从句的连接词。     worries the doctor most是主语从句，且从句缺少主语，要用what引导，排除B,D;      they could find the cause of the disease是表语从句，从句不缺主语和宾语，排除C；缺少状语，所以选A。