$28 \cdot 13$ In an experiment, a constant $c$ was measured to be 2.43865, with an error range of $\pm 0.00312$. The experimenter wants to publish the value of $c$, where each digit is a significant figure, meaning that regardless of the size of $c$, the published value of $c$ (with $n$ digits) must match the first $n$ digits of the true value of $c$. What is the most precise value of $c$ that the experimenter can announce?
(A) 2 .
(B) 2.4 .
(C) 2.43 .
(D) 2.44 .
(E) 2.439 .
(39th American High School Mathematics Examination, 1988)
［Solution］From the given conditions, we know

that $\square$
$$
\begin{array}{l} 
2.43865- 0.00312 \leqslant c \leqslant 2.43865+0.00312, \\
2.43553 \leqslant c \leqslant 2.44177,
\end{array}
$$

When rounded to one significant digit, $c=2$,
when rounded to two significant digits, $c=2.4$,
when rounded to three significant digits, $c=2.44$,
when rounded to four significant digits, $c$ is some number between 2.436 and 2.442. Therefore, the most precise value the experimenter can announce is 2.44. Hence, the answer is $(D)$.