参考答案：D

解析： accidentally意为“意外地，偶然地”，如：We accidentally found an ideal solution. 我们意外地找到了一个理想的解决办法。而initially意为“起初，开始”，如：Initially Alice opposed the plan, but she changed her mind later. 一开始爱丽丝反对这个计划，但后来她改变了主意。fortunately意为“幸运地”，如：Fortunately，nobody was killed in the accident. 幸运的是事故中没人遇难。frustratedly意为“受到挫折地，灰心丧气地”，如：He retried the method frustratedly, knowing that chances of success were slim. 他灰心丧气地又试了一次那个方法，知道成功的可能性很小。