单词详细解析：
urban.
adj.都市的；具有城市或城市生活特点的；市内
例句：
Most of the population is an urban population.
绝大多数人口都是城镇居民。
udder.
n.（牛、羊等的）乳房
例句：
The udder cell's genes took up residence in the egg and directed it to grow and develop.
乳腺细胞的基因在该卵子中安营扎寨，令其生长发育。
unite.
vt.（使）联合，合并，统一；并有，兼备；混合，化合
vi.（为某事）联合，联手，团结
例句：
The two parties have been trying to unite since the New Year.
这两个党从新年开始就试图联合。
upset.
vi.打翻，弄翻；打乱，打搅
vt.打乱，搅乱；推翻，弄翻；使心烦意乱；使翻倒
n.心烦意乱；翻倒，颠覆；混乱
adj.沮丧的；难过的；失望的；（肠胃）不舒服的.
例句：
The whole incident had upset me and my fiancee terribly.
整个事件搞得我和未婚妻都心烦意乱。
usually.
adv.通常，经常，平常，惯常地；一直,向来；动不动,一般
例句：
The best information about hotels usually comes from friends and acquaintances who have been there.
关于酒店的最可靠信息通常来自曾经去过那里的朋友和熟人。