核心考点：副词用法。 解题思路：'relatively' 是副词，修饰形容词或副词。句子B符合用法。 选项分析： A选项“The car is relatively speed compared to a bicycle.”：speed 是名词，不能用副词 relatively修饰。 B选项“The book is relatively long but very interesting.”：correct, 'relatively' 修饰形容词 'long'。 C选项“She sings relatively beautiful at the concert.”：beautiful 是形容词，应用副词 beautifully。 D选项“This cake is relatively delicious and sweet, so i will relatively enjoy it.”:第一个 relatively 修饰形容词delicious 正确，第二个位置错误，enjoy是动词不能用relatively修饰。 易错点：混淆relatively的使用对象（形容词和副词），忘记副词不能直接修饰名词和动词。