Kittens usually begin grooming themselves when they are about four weeks old. 19 five weeks of age, kittens also begin grooming other baby cats, as well as their mom. 20 grooming among cats, called allogrooming, often continues into adulthood.
答案：(A) At
【句型】at + 年齡/時間點 字詞搭配，At five weeks of age（在五週大的時候）；for、in、on 都不是這種表達特定年齡時間點的正確介系詞。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
四個都是最基本的介系詞，而講「五週大」在中文裡不需要介系詞，中文完全幫不上忙。這一格靠的是「介系詞配時間」的分工。
四個選項都是介系詞，可見這一格考的是搭配。空格後面是 five weeks of age（五週大），指的是一個明確的年齡點，而表示「在某個年齡」用 at：At five weeks of age（在五週大的時候）。同一組的說法還有 at the age of five（在五歲時）、at five（五歲時）、at birth（出生時）。順便把時間類介系詞的分工整理一次，這一組很常考：at ＋ 時間點或年齡（at five、at noon、at birth）；on ＋ 日期或星期（on Monday、on July 4）；in ＋ 月份、季節、年份（in July、in winter、in 2010）。範圍由小到大：at 最小、on 中間、in 最大。
For：表持續的一段時間（for five weeks 持續五週），但本句要說的是「在五週大這個時間點」，不是持續五週。In：配的是月份、季節或年份這種較長的區間，不配年齡點。On：配的是日期或星期，也不配年齡。
at five weeks of age（在五週大時）、at the age of（在……歲時）、at birth（出生時）、for ＋ 一段時間（持續……）、in ＋ 年份月份（在……）、on ＋ 日期星期（在……）。
When it comes to personal hygiene, cats are the best example of cleanliness. They are naturally equipped with 16 they need to groom themselves: a flexible tongue with which to lick, forepaws they moisten with saliva and use like washcloths, and teeth to dig out tougher pieces of food. 17, adult cats may spend as much as half of their waking hours grooming themselves, their relatives and friends.
Mothers begin licking their kittens 18 birth---to clean them, stimulate the release of body waste, wake them up to suck milk, and provide comfort. Kittens usually begin grooming themselves when they are about four weeks old. 19 five weeks of age, kittens also begin grooming other baby cats, as well as their mom.
20 grooming among cats, called allogrooming, often continues into adulthood. Allogrooming is a social activity that serves to strengthen the bond between cats.