题目内容：
根据中文提示，把对话中缺少的内容写在线上。这些句子必须符合英语表达习惯。打句号的地方，用陈述句；打问号的地方，用疑问句。
提示：Peter给Johnson教授打电话请病假，接电话的是教授的助手Sally。
(Peter=P；Sally=S)
P：Hello! This is Peter speaking． 5 1 ，please?
S：I’m sorry．Professor Johnson is not here at the moment．This is his assistant，Sally． 52 ?
P：Yes．Please tell the professor that l’ d like to ask for sick leave for tomorrow．
S：Sorry to hear that．May I ask 53 with you?
请在51处填写最佳答案。
答案解析：