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，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。
答對率 36%。
recommend 的中文是「建議」，你大概直覺想接不定詞（recommend to use）——但英文裡 recommend 後面要接動名詞或 that 子句，不接不定詞。
recommend + V-ing：The ADA recommends using a new toothbrush every three to four months.；或 recommend that + S + (should) + 原形：The ADA recommends that you (should) use a new toothbrush…。兩種都對，但不能寫成 recommend to use。
suggest、recommend、avoid、enjoy、finish、mind、practice、consider、admit——這些動詞後面一律接 V-ing。可以用一句話串起來記：avoid、enjoy、finish、mind、practice 都是「動作已經在進行或已經發生」的概念。
recommend + V-ing（建議做…）、suggest + V-ing（建議做…）、avoid doing（避免做）、consider doing（考慮做）、mind + V-ing（介意做）。
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.