29 age 21, he had already published 25 professional papers on that topic.
答案：(A) By
By age 21 意為「到二十一歲時（為止）」，與後面的過去完成式 had already published 搭配，表示在那個時間點之前已完成的事；In、With、On 都不能表達「到…為止」的截止概念。
答對率 31%。
表示時間的介系詞你背過 in／on／at，但 by 的「截止」概念常被忽略。by ＋ 時間點 ＝「到…之前、到…為止」，強調的是那個時間點之前累積完成的事：by then（到那時）、by the end of this year（到今年年底為止）、Finish it by Friday.（週五前完成）。
本句用了過去完成式 had already published 25 professional papers（已經發表了二十五篇論文）——過去完成式表示「在過去某個時間點之前就完成」，而那個時間點正是由 by age 21 標出來的。看到 had + p.p.，就往「by ＋ 時間點」想。
by 是「到…之前完成」（動作一次完成）；until 是「持續到…為止」（狀態持續）。Finish it by Friday.（週五前交）／Wait until Friday.（等到週五）。
by age 21（到二十一歲時為止）、by then（到那時）、by the end of…（到…結束前）、by the time…（到…的時候）、until（持續到…為止）。
Jean Piaget, a psychologist and pioneer in the study of child intelligence, was born in Switzerland in 1896. At age 10, he managed his first 26, a description of a partly albino sparrow he observed in a public park. At age 15, he 27 to devote his life to developing a biological explanation of knowledge.
He began his career as a zoologist, 28 mollusks and their adaptations to their environment. 29 age 21, he had already published 25 professional papers on that topic.
Later, after working with Alfred Binet in Paris, he became interested in levels of logic used by children taking standardized tests on intelligence. Piaget 30 to establish a body of psychology all his life and became a very influential figure in educational psychology. His works were all written originally in French and were later translated into English.