In the book of Ruth, what specific legal custom allowed Boaz to redeem Naomi's land and marry Ruth, thus ensuring the continuation of Elimelech's family line?

A The law of gleaning, which allowed the poor to gather leftover crops. B The kinsman-redeemer (goel) law, which obligated a close relative to redeem property and continue the family line. C The sabbatical year law, which required land to lie fallow every seventh year. D The levirate marriage law, which required a brother to marry his deceased brother's widow.