Hi. I am a question. What does "Re:" mean in this email?
Help me, please!

Derek's mother had a busy day ahead of her. she asked derek to by going to the store and buying groceries. "we need bread, milk, cheese and other things," she said. "don't forget to take the list. i know how forgetful you are." derek agreed to do the shopping, but after his mother left, he quickly forgot about his promise. several hours later, he remembered. he ran out the door and headed to the store. he forgot to take the shopping list. when he returned home, derek unloaded the groceries. he had carefully packed the dairy products and vegetables in one bag. the bread and canned goods were packed in the other. he reached into the first bag and took out beans, broccoli, butter and cheese. he put them neatly into the refrigerator. then he put away the bread and soup. when his mother came home, she opened the refrigerator and frowned. 1 select the correct answer. read the passage. then choose the best prediction of what will happen next. a. derek's mother tells him that he forgot to buy milk. b. derek's mother tells him he bought the wrong kind of milk. c. derek's mother tells him that she bought the milk.