In an Angle Class I occlusion, which cusp of which permanent tooth moves between the mesiolingual and distolingual cusps of the mandibular second molar in a working side movement?

Mesiolingual cusp of the maxillary second molar.

Distolingual cusp of the maxillary second molar.

Distolingual cusp of the maxillary first molar.

Mesiolingual cusp of the maxillary first molar.