When A and A are elongated, M is pulled by gravity and spring A. If the pulling force is equal to mg, spring B is in the original length. If the pulling force is less than mg, the spring B is in compression; If the pulling force is greater than mg, the spring B is in an extended state. So, A is wrong.

If B and A are compressed, M will be subjected to gravity, and spring A will exert downward elastic force on it, then spring B will exert upward elastic force on M, and then B will be compressed. Therefore, B is correct.

If C and B are elongated, then M is subjected to gravity, and B spring pulls down, then the elastic force of A spring on M must be upward, that is, A must be elongated. So, C is correct.

If D and B are compressed, M is acted by gravity, and B spring is elastic upward. If the elastic force is equal to gravity, the spring is in the original length; If the elastic force is greater than gravity, the spring is in compression; If the elastic force is less than gravity, the spring is in an extended state. So, d is wrong.

So choose BC.