Dropdown list - validate without VBA

Hi

I am creating a spreasheet that will require a user to select different options from a dropdown list.

Once option 'A' has been selected option 'B' will be populated and so on.

The problem I have is that if the user then goes back and changes an earlier option the other ones are not updated e.g. option 'B' will still hold the old value from the perious list.

So my question is I would like to know if it is possible to protect a cell once it has been changed or null a value in the corresponding cell if it is changed.

For example the user chooses an option in the first dropdown box which populates the options in the second list, they choose all the options required but then decide to change a value in the first dropdown list, this obvious means that all the other data will be invalid.

I have attached my spreadsheet to help.

Any advice is appreciated and it is worth pointing out that I am not allowed to use VBA but I am fully aware of how to solve this problem if I could