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Q1. Who are the two types of users that can grant sharing privileges on a given record?

From

Cover of Mastering Salesforce CRM Administration

Note

To grant access to a record, you must be one of the following users.
- The record owner
- A user in a role above the owner in the hierarchy (if your organization’s sharing settings control access through hierarchies)
- Any user granted “Full Access” to the record
- An administrator

Granting Access to Records
https://help.salesforce.com/articleView?id=granting_access_to_records.htm&type=0