为什么quiz 变成复数要双写z再加es?

quiz 报错
英[kwɪz]美[kwɪz]
n.考查；恶作剧；课堂测验
vt.挖苦；张望；对…进行测验
复数quizzes
过去式quizzed
过去分词quizzed
现在分词quizzing
例句：
AndIshould'vesaidthere's twoquizzesandafinalexam.
而且我应该已经提过了,一共有两次小测验和期末考试.
To further coaxtheminto the classroom,hegives his studentsbriefquizzesbeforeeachclass.
为了把学生“哄进”课堂,他在每节课上课前给学生一个简短的测验.