A step-down transformer operates at $100\%$ efficiency. Its input voltage is $240\,\text{V a.c.}$ and its output voltage is $60\,\text{V a.c.}$. The current in the primary coil is $0.50\,\text{A}$. Determine the current in the secondary coil.
- A$0.13\,\text{A}$
- B$0.50\,\text{A}$
- C$2.0\,\text{A}$
- D$8.0\,\text{A}$