What does the output x of the following MongoDB aggregation query result into; db.posts.aggregate( [ { $group: { _id; "$author", x: { $sum: $likes } } } ] )

What does the output x of the following MongoDB aggregation query result into; db.posts.aggregate( [ { $group: { _id; "$author", x: { $sum: $likes } } } ] )
A . Average of likes on all the posts of an author, grouped by author
B . Number of posts by an author
C . Sum of likes on all the posts by all the authors
D . Sum of likes on all the posts by an author, grouped by author

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Answer: D