Store your toothbrush in an upright position, and let 15 air dry.
答案：(A) it
【代名詞】it 代替前面提到的單數名詞 toothbrush，表示「讓它風乾」。one 泛指同類但非同一物；the 為冠詞非代名詞；which 為關係代名詞，不可單獨作受詞使用於此句構。
let 後面該接什麼你大概不會卡，真正讓人猶豫的是 it 與 one 好像都能代替前面的 toothbrush，一時分不出來。
這是指標勾環：空格要的是一個回指前面東西的代名詞。先看結構，let 後面要接受詞再接原形動詞，所以空格是 let 的受詞；再問這個受詞指的是誰——同一句前半的 your toothbrush，而且是「你自己那一支」這一支特定的牙刷，所以用 it。填進去讀成 let it air dry（讓它自然風乾）。鑑別度只有 16，是本批最低的一題，但它本身不難：let 後面接受詞再接原形動詞是課本教過的句型，各種程度的人都答得出來，題目自然分不出高低。
one 代替的是「同類當中的任何一個」（I need a pen. Do you have one?），這一句講的是你手上那一支，不是隨便一支；the 是限定詞，後面一定要接名詞，不能單獨站著當受詞；which 是關係代名詞或疑問詞，前面既沒有逗號也沒有問句結構，接不上去。
let ＋ 受詞 ＋ 原形動詞（讓某人或某物做某事）、air dry（自然風乾）、in an upright position（直立擺放）、it 與 one 的分工（it 指前面提過的那一個；one 指同類中的任一個）、store（存放、擺放）、share（共用）。
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.