$36 \cdot 23$ In a row with 120 seats, some seats are already occupied. If a new person comes and no matter where they sit, they will be next to someone, then the minimum number of people originally seated is
(A) 30.
(B) 40.
(C) 41.
(D) 60.
(E) 119.
(6th American Junior High School Mathematics Examination, 1990)
［Solution］As shown in the figure, every 3 chairs accommodate 1 person, that is, sitting on the middle chair (the shaded part in the figure) will suffice.

This way, at least $\quad 120 \div 3=40$ people need to be seated. Therefore, the answer is $(B)$.
Note When the number of seats is 121 or 122, the minimum number of people to be seated is 41.