Questões de Concurso Militar ESCOLA NAVAL 2016 para Aspirante, 2° Dia

Which question word best completes the paragraph below?

So if smoking is so bad for you, ______ is it so hard to quit? Stopping smoking is difficult for several reasons: nicotine is highly addictive, rewarding psychological aspects of smoking and genetic predisposition.

(Adapted from http://www.spine-health,com/wellness/stop-smoking)