Which source is NOT a valid data source for a mail merge? database
Outlook contacts
Excel worksheet
PDF document

database

Explanation:

From the different options listed below the one that is not a valid data source for mail merge is a database. This is because mail merge uses all the different Microsoft Word and Microsoft Excel types of documents, these include Outlook contacts , Excel worksheet , and even PDF document. Databases on the other hand are server-side programs that contain a wide range of information but are not considered to be a specific document type and therefore not accepted for mail merge.

i think it could be virus