A primaryid_c custom field exists on the candidate_c custom object. The filed is used to store each candidate's id number and is marked as Unique in the schema definition.
As part of a data enrichment process. Universal Containers has a CSV file that contains updated data for all candidates in the system, the file contains each Candidate's primary id as a data point. Universal Containers wants to upload this information into Salesforce, while ensuring all data rows are correctly mapped to a candidate in the system.
Which technique should the developer implement to streamline the data upload?

Refer to the following Apex code:

What is the value of x when it is written to the debug log?

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How can a developer implement this feature?