$35 \cdot 3$ A domino is composed of two squares, one black and one white, arranged like this: 民. Among the following chessboards, which one cannot be completely covered without repetition using only this type of domino?
(A) $3 \times 4$.
(B) $3 \times 5$.
(C) $4 \times 4$.
(D) $4 \times 5$.
(E) $6 \times 3$.
［Solution］Dominoes consist of one white square and one black square, so the figures they can completely cover should have the same number of black and white squares.
However, in (B), the number of black and white squares is different.
Of course, we can also consider the parity of the number of small squares, $(B)$ is the only figure with an odd number of small squares.
Therefore, the answer is $(B)$.