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Explanation:

Let the length of track is d metres.

$$\begin{gathered} \therefore \frac{A}{B}=\frac{d}{d-12} \\ \frac{A}{C}=\frac{d}{d-18} \\ \frac{B}{C}=\frac{d}{d-8} \end{gathered}$$

Solving we get, d = 48 m

Hence, option (b).

Alternatively,

When A finished, the gap between B and C was 6 m. After B ran another 12 m, the gap is increased by another 2 m.

∴ A gap of 2 m is created when B runs 12 m.

∴ A gap of 8 m is created when B runs = 12 × 4 = 48 m