"The" vs "A"

[vcolts]

There is a section in the TOEIC Speaking & Writing Test where you are supposed to describe a picture.

Let's say there is a picture of a man holding a cell phone at a refinery (an example question from the internet).

I would write: A worker is holding a cell phone at a refinery.

But the example answer always had "the" instead of "a" (as in "the" worker is holding a...)

I think both articles suffice for answering, right?

Thanks in advance.

[Tdol]

I would follow the pattern they use, but the indefinite article is OK there.