Many Americans replace their toothbrushes only once or twice a year. The ADA, however, recommends 12 a new toothbrush every three to four months.
答案：(C) using
recommend 後面接動名詞當受詞（recommend + V-ing），故填 using；use 是原形，to use 是不定詞，used 是過去式或過去分詞，都不能直接當 recommend 的受詞。
recommend 的中文是「建議」，你大概直覺想接不定詞，寫成 recommend to use。但英文裡 recommend 後面要接動名詞或 that 子句，不接不定詞。
這一題考的是這個動詞要求後面跟什麼形式，屬於用法而非字義，句法一關就能定案。空格前是主要動詞 recommends，空格處要填的是它的受詞。recommend 的兩種正確用法是：recommend + V-ing（The ADA recommends using a new toothbrush every three to four months.）以及 recommend + that + S + (should) + 原形 V（The ADA recommends that you should use a new toothbrush…）。本句空格後面沒有 that、也沒有主詞，只有名詞片語 a new toothbrush，所以走的是第一種，填 using。驗算法：把兩種寫法都試著唸一遍，能唸通的那一種就是句子要的形式；若填的字讓 recommend 後面既不是動名詞也不是完整子句，那一定不對。這一組要接動名詞的動詞值得一起記：suggest、recommend、avoid、enjoy、finish、mind、practice、consider、admit。與它們對照的是慣接不定詞的那一組：want、hope、plan、decide、promise、agree——這一組多半帶著「還沒做、打算去做」的意味。兩組分開記，寫作時最不容易出錯。
use 是原形動詞，一個句子已經有 recommends 這個主要動詞，不會再直接接一個原形動詞。to use 是不定詞，形式上像個受詞，但 recommend 不收不定詞當受詞，這是這個字的用法規定，記下來即可。used 有兩個身分——過去式與過去分詞，兩種都當不了 recommends 的受詞：當過去式會讓句子出現第二個限定動詞，當過去分詞則需要有名詞可修飾，本句沒有。
recommend + V-ing（建議做…）、recommend + that + S + (should) + V（建議…）、suggest + V-ing（建議做…）、avoid + V-ing（避免做）、consider + V-ing（考慮做）、mind + V-ing（介意做）、decide + to V（決定做）。
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.