明明每天坚持背英语单词，他建立了英语单词错题本文件“mistakes.txt”，将每天记错的单词增加到该文件中，下列打开文件的语句最合适的是？（ ）
f = open("mistakes.txt")
f = open("mistakes.txt","r")
f = open("mistakes.txt","a")
f = open("mistakes.txt","w")