Discussion
Let the set A ={1, 2, 3, 4, 5, 6, 7, 8, 9, 10}. Then number of subsets of A containing exactly two elements is
Answer: C
There are 45 such subsets.
A two-element subset is a choice of 2 elements from 10, with order not mattering:
\[ \binom{10}{2} = \frac{10 \times 9}{2} = 45 \]
90 counts ordered pairs, so each subset is counted twice. 20 and 40 do not come from any correct count.
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