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There cant be any Many to One relationship from my concern but If you take it the either way .. all One to Many Relationship are Many -to-One From the Other side . if you create a Master Detail Relationship taking A as master and B as detail then you can see that for every A there is a number of B records which is One to Many and same looking from the B side we can say that for each B we can have many A .we can select any of the A records which comes to be Many to One .

To create a many to one  relationship, create a new custom field of type "Look Up" on the child object (the many side of the relationship) to the parent object (the "one" part of the relationship).

There are 3 types of relationships that can be created in Salesforce  master-detail, lookup, and hierarchical.  Each of these relationships is a one to many relationship.

Master-Detail

    A detail record must be linked to a master record in order to save the record (e.g. you must link an opportunity to account to save the opportunity).
    Only one level deep – a detail object cannot also be a master object.  Please vote to remove this limitation: Multiple Master-Detail relationships.
    A detail record can have two different master records (two master-detail relationships must be created).
        
Lookup Relationship

    One to Many relationship links one record to another record.
    Can link standard to custom objects and vice versa, or link to the same object type (parent account, for instance).

Hierarchical

    A lookup relationship on the user object to the user object (for instance the standard field “Managerâ€�).
    Record cannot point to itself (I cannot be my own manager).

Important :

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