20．Cancer is a disease ___causes are still not clearly known．（　�。�
A．whoseB．whichC．thatD．who

分析 癌癥這種疾病的起因現(xiàn)在仍然不清楚．

解答 答案：A
題干中causes are still not clearly known是修飾disease的一個定語從句；從句中主語causes缺少表示所屬關系的限定詞，要用whose來連接．故選A．

點評 定語從句是一類比較重要的復合句．定語從句關系詞的選擇要根據(jù)從句的結構來確定，從句如果缺少主語或賓語，要用關系代詞；從句結構如果完整，要用關系副詞．學習時，要注意區(qū)分和掌握．

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