你的答案错了,不信去你老师.
这里正确填法是【became】
有个when的通常情况是一般现在时.
填过去分词的话,
when 需改为since 【自从】
His cousin had become a lawyer since he was twenty-four.
还有填过去分词的另一种情况,就是有ago.ago前面就填时间长短,不能填他几岁了.
His cousin had become a lawyer three years ago
【他的表哥三年前就成了一名律师.】这里three years是我自己加的
还有忘了说了,become 的过去式是became,过去分词又变成become