(ESCOLA NAVAL- 2016)Which question word best compl

(ESCOLA NAVAL - 2016)

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)