Answer:
D. source has more than three authors.
Explanation:
'et al.' is used in the in-text citations where three or than than three authors are to be mentioned for a particular work. The meaning of the Latin word 'et al.' is 'and others'. The last name of the first author is written and then 'et al.' is added to it. A comma (,) is then added along with the year of the publication of the work. In this way, the in-text citation is shortened by just adding the last name of the authors.
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