Hi! Are both acceptable? Thanks, Fulvio
1. What is the tallest tower in the world?
2. Which is the tallest tower in the world?
" at the beginning. The question asks you to identify something in a open set of towers; that's the cue that "what" will probably be used. Contrast that with "There's a tower in London and a tower in Rome.
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Because of "in the world", our tendency is to choose "What?" at the beginning. The question asks you to identify something in a open set of towers; that's the cue that "what" will probably be used.
Contrast that with "There's a tower in London and a tower in Rome. Which tower is taller?"
which selects; what identifies.
CJ