Quick question on Epson R2400

I have just purchased a rather cheap but good condition R2400, with the aim to use it for B&W printing only (I have a Canon S9000 for colour).

My question is due to the expensive nature of the original Epson inks, if I select B&W only, which inks actually do the printing, I.e. Is it just the photo black, light black and light light black or a combination of all the 8 colours ?

Thanks
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