It seems like "which is" is more appropriate than "what's." What do you think?
"Which" and "what" are both very similar and can often be used interchangeably and have the same meaning. The main difference is that "which" is mostly used when someone is asking about something with a limited set of options that are already defined or known, whereas "what" can be used when the options are not known. "What" is also generally a little less formal than "which". Ex: Which shoes should I wear tonight, the yellow ones or the red ones? Ex: What shoes should I wear tonight? In the case of the video, you are right about "which" being the better option. Since he defines the two options and makes them known, "which" would sound better in this case. However, "what" is still acceptable here even though it doesn't sound as good as "which".
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