Small Air law Q

[Cron]

I would be most grateful for a suggested answer to the following Q and comments on the comments.

Which of the following transponder codes is not a valid SSR mode A squawk?
a) A/0000
b) A 5678
c) A/7700
d) A/7777

I have information saying 0000 should not be used but other info saying it is for malfunction.

[Flying Farmer]

Ermmmmm the available numbers are from 0 to 7, can you figure it yet :}

no books to hand but I think 0000 is for a military escort.

[Cron]

... the available numbers are from 0 to 7 ..

That's the bit I needed. Thank you FF.

[helicopter-redeye]

Its B in case its not clear yet.

(NB on the ATIS on NDB question I put it to a high hours pilot/ CPL/ IR/ Instructor today while flying and he could not think of any ATIS on NDB in JARland either)

[Cron]

Thanks HR - I'm rather getting to like this Air Law.

[A320rider]

none of them, if you don't have a transponder in your Ultralight!!;)