Ans: D
The compound equation is equivalent to $\dfrac{4}{5a}=\dfrac{5}{7b}$ and $\dfrac{5}{7b}=\dfrac{7}{9c}$. Then we have
The compound equation is equivalent to $\dfrac{4}{5a}=\dfrac{5}{7b}$ and $\dfrac{5}{7b}=\dfrac{7}{9c}$. Then we have
$\begin{array}{rcl}
\dfrac{4}{5a} & = & \dfrac{5}{7b} \\
\dfrac{b}{a} & = & \dfrac{25}{28} \\
b:a & = & 25 : 28 \\
a:b & = & 28 : 25 \ \ldots \unicode{x2460}
\end{array}$
Also, we have
$\begin{array}{rcl}
\dfrac{5}{7b} & = & \dfrac{7}{9c} \\
\dfrac{c}{b} & = & \dfrac{49}{45} \\
c:b & = & 49 : 45 \\
b:c & = & 45 : 49 \ \ldots \unicode{x2461}
\end{array}$
By combining $\unicode{x2460}$ and $\unicode{x2461}$, we have
$\begin{array}{ccccccccccc}
a & : & b & & & = & 28 & : & 25 & & \\
& & b & : & c & = & & & 45 & : & 49 \\ \hline
a & : & b & & & = & 28 \times 9 & : & 25\times 9 & & \\
& & b & : & c & = & & & 45\times 5 & : & 49 \times 5 \\ \hline
a & : & b & : & c & = & 252 & : & 225 & : & 245
\end{array}$
Therefore, $b < c < a$.
