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I normally finish work at 5:00, but this week I ___________ (a. have worked, b. am working) until 6:00 to earn a bit more money. Hi,

I have taken the example given above from an online exercise. According to the site at which the exercise is given, the correct option is
b (present continuous), I'm wondering if
a (present perfect) is possible as well. Can't we also use the
present perfect (have worked) there, teachers, for example, in a context where the speaker is in the middle of the week, or, for example, if the week is nearly finished/complete at the time of speech?
Thank you.