单选题 When an organism is completely encapsulated and preserved, it becomes a fossil, therefore turning into evidence of things that once lived.

【正确答案】 A

【答案解析】[解析] therefore和thereby均为副词，表“因此”，但therefore后通常跟一个句子；而thereby后常接现在分词作状语。as a result of“由于”(as a result才表“因此”)，因此A为答案。