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Difficult

Non-consecutive Numbers

COMBIN-XNECJ6

How many 4-element subsets of the first 20 positive integers contain no pair of consecutive numbers?

A

${16\choose 4}$

B

${17\choose 4}$

C

${20\choose 4}-15$

D

${20\choose 4}-4$

E

${20\choose 4}$