Which of the following statement is NOT correct?

Options

1. AGround wave signals are more stable than the sky wave signals.
2. BThe critical frequency of an ionospheric layer is the highest frequency that will be reflected back by the layer when it is vertically incident.
3. CElectromagnetic waves of frequencies higher than about $30 \mathrm{MHz}$ cannot penetrate the ionosphere.
4. DSky wave signals in the broadcast frequency range are stronger at night than in the day time.