He remembered a kind of glue developed by a colleague that everyone thought 18 a failure because it did not stick very well.
答案：(B) was
過去式 was，配合整篇故事的過去式敘事，「大家都認為這（膠水）是失敗品」，是回憶過去的看法；is（現在式）、will be（未來式）、has been（現在完成式）都不符合過去式敘事語境。
讀到 that everyone thought ___ a failure，你很自然會把 that 當成 thought 的受詞（「大家所認為的那件事」），這樣一來空格前面就沒有主詞了，於是四個選項怎麼填都覺得怪。實際上 that 是空格這個動詞的主詞，everyone thought 只是插進來的插入語。
先把插入語遮起來，句子剩下 a kind of glue ... that ___ a failure，還原成完整子句就是 The glue was a failure.（這種膠水是失敗品），everyone thought 是外加的「大家認為」。這個遮蓋動作，和 104 年第 18 題把 forming an exercise system 括起來、108 年第 17 題把兩個逗號之間的成分遮掉，是同一個閱讀動作。句子結構清楚之後就只剩時態：同一句的 did not stick、前一句的 remembered、後一句的 coated 與 found 全是過去式，整篇在講一段已經結束的往事，所以用 was。
is 是現在式，會變成「大家現在認為它是失敗品」，但這款膠水後來做成了 Post-it，語意剛好相反。will be 是未來式，與回憶敘事不合。has been 是現在完成式，強調「到現在為止一直都是」，同樣與後來翻案成功的結局牴觸——這四個選項其實是在測你有沒有讀懂整篇故事的時間軸。
a failure（失敗的人或事物）、turn out to be a failure（結果以失敗收場）、stick well（黏得牢）、develop a product（研發產品）、a colleague of mine（我的一位同事）、think of A as B（把 A 看作 B）
Art Fry was a researcher in the 3M Company. He was bothered by a small irritation every Sunday as he sang in the church choir. That is, after he 16 his pages in the hymn book with small bits of paper, the small pieces would invariably fall out all over the floor.
One day, an idea 17 Art Fry. He remembered a kind of glue developed by a colleague that everyone thought 18 a failure because it did not stick very well. He then coated the glue on a paper sample and found that it was not only a good bookmark, but it was great for writing notes. It would stay in place 19 you wanted it to. Then you could remove it 20 damage.
The resulting product was called the Post-it, one of 3M's most successful office products.