单选题 Brushing removes larger particles, but dentists suggest brushing the back of the tongue as well, where food residues and bacteria ______.

【正确答案】 D

【答案解析】[解析] D项“congregate聚集”，根据空格处的意思：舌头后面是食物残渣和细菌聚集处，故选D。A项flourish繁荣；B项collaborate合作；C项embark着手，均不符合题意。