$36 \cdot 1$ In the $4 \times 4$ square shown in the figure, the numbers $1,2,3, \cdots, 15,16$ are written. The square is folded in the following four steps:
(1) The top half is folded over the bottom half.
(2) The bottom half is folded over the top half.
(3) The right half is folded over the left half.
(4) The left half is folded over the right half.

After these four folds, the number on top is
(A) 1 .
(B) 9 .
(C) 10 .
(D) 14 .
(E) 16 .
(7th American Junior High School Mathematics Examination, 1991)
[Solution] After the first two steps, the last row has been moved to the top; the third row is at the bottom.

When steps (3) and (4) are performed, only the changes in the third row need to be considered. The result is that 9 is on top.
Therefore, the answer is $(B)$.