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〉}