13.4 Given that $a, b, c, d$ are integers, and $a<2b, b<3c, c<4d$. If $d<100$, then the maximum value of $a$ is
(A) 2367.
(B) 2375.
(C) 2391.
(D) 2399.
(E) 2400.
(40th American High School Mathematics Examination, 1989)
[Solution] Given that $a, b, c, d$ are integers, we have
$$
a \leqslant 2 b-1, \quad b \leqslant 3 c-1, \quad c \leqslant 4 d-1, \quad d \leqslant 99 \text {, }
$$

Therefore, $a \leqslant 2(3 c-1)-1 \leqslant 2[3(4 d-1)-1]-1$
$$
\leqslant 2\{3[4 \cdot(99)-1]-1\}-1
$$

Thus, $a \leqslant 2367$, so the maximum value of $a$ is 2367.
Hence, the answer is $(A)$.