Tom, Bill and Dave lost their backpacks. They are at the Lost and Found desk. The backpacks are the same, but the things in the right backpack. Tom: I have a math book and a pencil case in my backpack. There are three pencils, a pen and an eraser in the pencil case. Bill: I have a Chinese dictionary, a math book and two notebooks in my backpack. Dave: There are two CDs, three picture books in my backpack. My English book is also in it. Which of the following(下面)is in Tom's backpack ? A(an)________.