Which statement is incorrect?
- AThe speed of long-wavelength infra-red radiation in a vacuum is greater than that of short-wavelength ultraviolet light.
- BThe speed of microwaves in air is approximately $3.0 \times 10^8\,\text{m s}^{-1}$.
- CThe speed of $\gamma$-rays emitted from a sample of cobalt-60 is $3.0 \times 10^5\,\text{km s}^{-1}$.
- DThe X-rays emitted in a supernova explosion reach the Earth at the same time as the visible light emitted.