EEAR 2016/2

Read the text and answer question.

[1] Thomas Edison tried two thousand different materials
in search of a filament for the light bulb. When none
worked satisfactorily, his assistant complained, “All our
work is in vain. We have learned nothing.”
[5] Edison replied very confidently, “Oh, we have come a long
way and we have learned a lot. We know that there are two
thousand elements which we cannot use to make a good
light bulb.”
(Adapted from academictips.org)