A. {〈1,1〉,〈1,3〉,〈2,2〉,〈2,4〉}

B. {〈1,2〉,〈2,1〉,〈2,3〉,〈3,4〉}

C. {〈2,1〉,〈1,2〉,〈3,2〉,〈4,3〉}

D. {〈1,2〉,〈2,3〉,〈3,4〉,〈2,4〉}