GMATPrep PP (PS排列組合)

[lucyyeh]

請問以下題目該怎麼做?謝謝!
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There are 10 children in a company’s day-car center, and a pair of children is to be selected to play a game. At most, how many different pairs are possible?
A) 100
B) 90
C) 50
D) 45
E) 25
答案 D

[Frankts]

C10取2 好像就出來了