# Propagation

### Explanation of question 29

- i The heights of the ionospheric layers are constant
- ii The D-layer is always present

1. i and ii are both true
2. i is true but ii is false
3. i is false but ii is true
4. i and ii are both false

The answer is :- 4 The height of the ionospheric layers varies not only seasonally, but daily; the distances normally quoted above earth are therefore only approximate. lonisation of the D-layer ceases at night.

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