There are 10 students in a room. If every student shakes hands with every other student exactly once, how many handshakes will there be in total?

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• A 100
• B 90
• C 45
• D 20

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Answer: C. 45

The first person shakes 9 hands, the second shakes 8 (already shook the first), the third shakes 7...
.So 9+8+7+6+5+4+3+2+1 = 45

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