A team of 3 is to be selected from 5 people (A, B, C, D, E).
- A and B cannot be together.
- C and D must be together.
- E cannot be with D.
- Step 1: Apply conditions with A. A is in the team. A and B cannot be together, so B is out.
C and D must be together, so include C, D. E cannot be with D, so E is out.
- Step 2: Form team. Team must be A, C, D (since B, E are excluded and team size is 3).
- Step 3: Verify. A, C, D: A not with B (valid), C with D (valid), E not with D (valid).
- Step 4: Check options. Options: (1) B, C (B invalid), (2) C, D (valid), (3) D, E (E invalid), (4) B, E (B, E invalid).
- Step 5: Conclusion. Option (2) is correct.





For any natural number $k$, let $a_k = 3^k$. The smallest natural number $m$ for which \[ (a_1)^1 \times (a_2)^2 \times \dots \times (a_{20})^{20} \;<\; a_{21} \times a_{22} \times \dots \times a_{20+m} \] is: