$36 \cdot 31$ A student observes during a $e$-day vacation:
(1) It rains seven times, either in the morning or in the afternoon;
(2) When it rains in the afternoon, the morning is clear;
(3) There are five clear afternoons in total;
(4) There are six clear mornings in total.
Then $e$ equals
(A) 7.
(B) 9.
(C) 10.
(D) 11.
(E) 12.
(10th American High School Mathematics Examination, 1959)
［Solution］Let the number of days corresponding to the weather conditions be as in the table below:
\begin{tabular}{c|c|c}
\hline & Rainy in the morning & Sunny in the morning \\
\hline Rainy in the afternoon & $a$ days & $b$ days \\
\hline Sunny in the afternoon & $c$ days & $d$ days \\
\hline
\end{tabular}

From the given conditions, we know that $a+b+c=7, a=0, c+d=5, b+d=6$. From the above, we can solve that $d=2$. Therefore, $e=a+b+c+d=9$. Hence, the answer is $(B)$.