设X = {1, 2, 3, 4}，Y = {a, b, c}，Z = { e, f, g} R是X到Y的关系且R={〈 2,b〉,〈 3,b 〉,〈 4,a 〉} S是Y到Z的关系且S={〈 a, e〉,〈 a, f〉,〈 b,g 〉,〈 c,g 〉}

题目：设X = {1, 2, 3, 4}，Y = {a, b, c}，Z = { e, f, g} R是X到Y的关系且R={〈 2,b〉,〈 3,b 〉,〈 4,a 〉} S是Y到Z的关系且S={〈 a, e〉,〈 a, f〉,〈 b,g 〉,〈 c,g 〉} 则R◦S 为（ ）

A. {〈 2,g 〉,〈 3,g 〉,〈 4,f 〉}

B. {〈 2,g 〉,〈 3,g 〉,〈 4,e 〉,〈 4,f 〉}

C. {〈 2,g 〉,〈 3,g 〉,〈 3,e 〉,〈 4,f 〉}

D. {〈 2,g 〉,〈 3,g 〉,〈 4,e 〉,〈 3,f 〉}

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