Low-level nuclear waste can be disposed of in a landfill.
This is best explained through the use of an optics diagram, this is a little too complicated to explain in a short answer, and as I can't draw an appropriate diagram in this answer, I will point you to this excellent resource which explains what you have asked very well!
Go onto the BBC website (you should have access to it even if you aren't in the UK) and paste this after the BBC url,
/bitesize/intermediate2/physics/waves_and_optics/image_formation_from_lens/revision/1/
90 kilometers because you need to multiply 40 by 2 and then you get 80 and finally you add 10 and get 90 kilometers
Option A looks like the best definition