Not sure if this should be posted in deals or Vouchers but rustlers are currently doing a competition to get a free voucher for buy 1 get 1 free on rustler burgers. The answer to the question is B - san siro.
Sainsburys currently have normal rustler burgers for £1 (quarter pounder, chicken and rib at my local one) so you can bag 2 for £1, 50p each which is a bargain.
The voucher can be used on any rustler or sub.
You don't need to download any software to print the voucher as it is emailed as a PDF and the voucher is valid until jan/feb next year so you can save and print more off as and when needed.