The term “waste gases” here refer to the fact that gases that
must be allowed to leave due to thermal expansion and pressure
differences – bear in mind that new air is constantly being pumped
into blast furnace because additional oxygen and carbon dioxide is
continuously required for this industrial process (to react with C
to generate CO).
In which case, the “waste gases” that must be allowed to leave
naturally includes N2, CO and CO2 (and
naturally some, but relatively little, of unreacted oxygen,
H2O vapour, noble gases, oxides of nitrogen,
oxides of sulfur, etc etc), even though these may not be “wastes”
in the sense of the word.
So, as far as this particular question is concerned (two
additional relevant points : ‘O’ level questions are simplified, so
it’s far easier to simply ask the student summarily thusly, rather
than an essay discussion on every subpoint; even Cambridge exam
questions may be argued to have some aspects of flaw or fault in
regards to interpretation), then the best answer would indeed be :
CO, CO2 and N2.
Although one might argue that oxides of nitrogen and sulfur are
more polluting than N2, but these are not present in significant
amounts.
Bottomline : the question isn’t actually asking “what are the
waste gases from the process occurring in the blast furnance?” but
is asking “what are the main chemical gases present in the waste
gases released from the blast furnace?”
See the difference?