frontier

n.国境，边境；边疆；尖端，新领域

frog, frog-march, frog-spawn, frogman, frolic, frolicsome, from, from memory, from now on, from side to side, from then on, from time to time, from...to, frond, front, front man, front runner, front-page, frontage, frontal, frontiersman, frontispiece, frost, frost-bite, frost-bitten, frostbound, frostily, frostiness, frosting, frosty, froth, frothily, frothiness, frothy, frown, frowsty, frowsy, frowzy, froze, frozen,