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dependence is marked first by an increased tolerance, with more and more of the substance required to produce the desired effect, and then by the appearance of unpleasant withdrawal symptoms when the substance is discontinued.

参考答案：

要点：句子框架：dependence is marked first by…, with…, and then by…。句子的难点：有并列从句和省略。with引导的是伴随结构作状语，修饰的是整个第一分句描述的状态。and引导的分句省略了谓语动词marked ，所以要注意其实该分句并不隶属于第一分句，而是与之并列的结构。

参考译文：依赖的最初表现为耐受力增强，用量越来越大才能达到预期效果，一旦停用就会产生不舒服的症状。

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