6. Questions 18-24

The population of a small country is organized into five clans-N, O, P, S, and T. Each year exactly three of the five clans participate in the annual harvest ceremonies. The rules specifying the order of participation of the clans in the ceremonies are as follows:
Each clan must participate at least once in any two consecutive years.
No clan participates for three consecutive years.
Participation takes place in cycles, with each cycle ending when each of the five clans has participated three times. Only then does a new cycle begin.

No clan participates more than three times within any cycle.

(18). If the clans participating in the first year of a given cycle are N, O, and P, which one of the following could be the clans participating in the second year of that cycle?
(A) N, O, S
(B) N, O, T
(C) N, P, S
(D) O, P, T
(E) O, S, T

(19). Which one of the following can be true about the clans' participation in the ceremonies?
(A) N participates in the first, second, and third years.
(B) N participates in the second, third, and fourth years.
(C) Both O and S participate in the first and third years.
(D) Both N and S participate in the first, third, and fifth years.
(E) Both S and T participate in the second, third, and fifth years.

(20). Any cycle for the clans' participation in the ceremonies must be completed at the end of exactly how many years?
(A) five
(B) six
(C) seven
(D) eight
(E) nine

(21). Which one of the following must be true about the three clans that participate in the ceremonies in the first year?
(A) At most two of them participate together in the third year.
(B) At least two of them participate together in the second year.
(C) All three of them participate together in the fourth year.
(D) All three of them participate together in the fifth year.
(E) None of them participates in the third year.

(22). If, in a particular cycle, N, O, and S participate in the ceremonies in the first year, which one of the following must be true?
(A) N participates in the second and third years.
(B) O participates in the third and fourth years.
(C) N and O both participate in the third year.
(D) P and T both participate in the fifth year.
(E) S and T both participate in the fifth year.

(23). If , in a particular cycle, N, O, And T participate in the first year and if O and P participate in the fourth year, any of the following could be a clan that participates in the third year EXCEPT
(A) N
(B) O
(C) P
(D) S
(E) T

(24). If, in a particular cycle, N, O, and S participate in the ceremonies in the first year and O, S, and T participate in the third year, then which one of the following could be the clans that participate in the fifth year?
(A) N, O, P
(B) N, O, S
(C) N, P, S
(D) O, P, S

(E) P, S, T

KEYS: ECAADCE