Indicate the rules that must be obeyed when designing supports of tooth-mucosa-supported removable partial dentures, in order to avoid surpassing the tolerance threshold of periodontal tissues:

1) supports should only involve the teeth adjacent to the edentulous space;
2) supports should involve the maximum possible number of teeth;
3) rests should be placed on the surfaces perpendicular to the acting force;
4) for esthetic reasons, the rests should be placed on the proximal and lingual surfaces;
5) rigid connection between the support and the denture base;
6) flexible connection between the support and the denture base;
7) supporting elements evenly located in the dental arch;
8) in the case of a small number of abutment teeth and tendencies toward periodontal disease, involving vast basal seat with the denture base;
9) for esthetic reasons, the support must not be designed on incisors.
The correct answer is: