Due to the profitable but unlawful practice of selling elephant tusks, elephants have been hunted to near extinction by unlicensed poachers. Ttherefore, an efficient way to prevent the hunting of elephants would be to file down the tusks from time to time, thus removing any incentive for hunting elephants. Which of the following is an assumption required by the argument? ['Elephants are only valued by poachers for their tusks.', 'Poachers often steal baby elephants to raise in captivity and later sell them for a profit.', 'Elephants whose tusks have been trimmed will be unable to defend themselves against poachers.', 'The demand for elephant tusks will decrease as supply dwindles.']