- For (a): \( \frac{18}{42} \equiv \frac{3}{7} \) is True.
- For (b): \( \frac{18}{49} \equiv \frac{5}{14} \) is False.
- For (c): \( \frac{9}{25} \equiv \frac{140}{500} \) is False.
Thus, the final answers are:
\[
\boxed{\text{(a) True, (b) False, (c) False}}
\]