These germs come from the mouth and can 11 in toothbrushes over time.
答案：(A) accumulate
accumulate in toothbrushes（在牙刷上累積）表示這些細菌會隨著時間在牙刷上累積；crumble（崩解）、establish（建立）、radiate（發散）均不符合此處語意。
四個選項都是動詞，中文擺進去好像都能講通，於是憑語感挑一個。accumulate 這個字，你的印象大概停在「累積金錢、累積經驗」這種抽象的東西上，不太會想到細菌也會 accumulate。
答對率五成二，鑑別度高達 61，是本批最會分人的題目之一，分的是有沒有把句尾的 over time 當成線索用。先看空格前後：主詞 These germs 回指前一句的 bacteria，後面是 in toothbrushes over time。over time（隨著時間過去）點明這是一個「量愈來愈多」的過程，四個選項裡只有 accumulate（累積、聚積）講得出這種隨時間變多的意思。填進去讀成 germs can accumulate in toothbrushes over time（細菌會隨時間在牙刷裡愈積愈多），也正好接得上前一句的 toothbrushes can harbor bacteria（牙刷會藏細菌）。
establish（建立）是及物動詞，後面一定要有受詞（establish a system），這裡 can ______ in toothbrushes 沒有受詞，形態上就接不起來；crumble（碎裂成小塊）的主詞通常是餅乾、牆壁這類本身會碎掉的東西，細菌不會碎；radiate（向外發散、輻射）講的是光、熱、情緒往外散，方向與「聚在牙刷裡」相反，而且它習慣配 from 而不是 in。三個都是動詞原形，形態吻合，容易被順手選走。
accumulate（累積、聚積）、over time（隨著時間過去）、harbor bacteria（藏有細菌）、crumble（碎裂成小塊）、radiate from（從……向外發散）、take care of（照顧、保養）。
Brushing your teeth regularly will help you maintain a healthy smile. But that smile won't last long if you don't take proper care of your toothbrush and switch to a new one often. According to the American Dental Association (ADA), toothbrushes can harbor bacteria. These germs come from the mouth and can 11 in toothbrushes over time.
Many Americans replace their toothbrushes only once or twice a year. The ADA, however, recommends 12 a new toothbrush every three to four months. Children's toothbrushes may need to be changed more 13 .
During those three to four months of use, there are several ways to keep a toothbrush clean. 14 , rinse your toothbrush thoroughly with tap water after use, making sure to remove any toothpaste and debris. Store your toothbrush in an upright position, and let 15 air dry. Most importantly, do not share toothbrushes.