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You can use "which" where the reference is indirect, e.g.
1. "I measured the twins yesterday." "And which (one) was taller?"
2. "I measured the twins yesterday. Which would you say is taller?"
3. "I know that one of your sons is at Eton; but I can't remember which."
You can use "which" where the reference is indirect, e.g.
1. "I measured the twins yesterday." "And which (one) was taller?"
2. "I measured the twins yesterday. Which would you say is taller?"
3. "I know that one of your sons is at Eton; but I can't remember which."
Who / Which 
Linear Mode
