You are given a paragraph (Passage), a question (Question) and two answer options (Option1 and Option2). Your task is to choose the correct answer to the question based on the passage. Answer of the question cannot be found directly from the passage, it is indirectly indicated in the passage. Do not answer with words other than the two options, and do not answer with option titles(e.g. "option1" or "1" are not acceptable answers.)

Passage: My best friend had her birthday coming up and I decided to throw her a surprise birthday party . I asked her mother for a list of people she would want invited and then I went to the store to pick out invitations . I looked at several different invitations . I decided on the pink polka dot initiations because my friend 's favorite color is pink . When I got home I filled in all of the information about the party . The party was going to be in one week at my house . I made sure to let everyone know it would be a surprise party so my friend would n't find out about the party by accident . After I was finished doing that , I went to the post office to mail out the initiations . Question: what was the stamp put on? Option1: invitation Option2: envelope
envelope