Slope of vertical lines (Quant Tickbox Quiz #8 Question 6)

Hey everyone! Can someone please help me on this?:

I am in doubt about this question, because in the prep swift videos, it clearly says that the slope of a vertical line is positive infinity.

Or does GRE officially say the slope is undefined??

I think GRE considers this to be not defined that is why it shows that the answer is incorrect but still good to check with the experts.

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This is correct.