Answer: C(n,k).
- A n<sup>k</sup>
- B C(n,k)
- C k!/(n-k)!
- D 2<sup>n</sup>
Correct answer: B. C(n,k)
Explanation: A binary string of length n with exactly k ones: choose k positions (out of n) to place the 1s; the rest become 0s. Number of ways = C(n,k).
Counting tree for selecting 2 items from {A, B, C} without repetition: 3 x 2 = 6 ordered arrangements.
Concept context
Counting, arrangements, and selections