Which one is it that or which, Help? The envelope that / which contained the winner's name was given to the host?

“you meet new people on a job. kayla morrison says, 'i work with two college students from the east coast, and it's been interesting learning about their lives.' kayla has also come to think of the café owner as a mentor who is providing her with valuable information about the food service industry. kayla is even thinking about applying for the culinary arts program at her high school.” what is the topic sentence of this paragraph?