Is your schoolbag too heavy to carry? The e-schoolbag will help you. In fact, the e-schoolbag should perhaps be called an e-textbook. It is a small computer for students. Heavy schoolbags have been a serious problem for them for a long time. But the e-schoolbag will change everything. It is much lighter than a usual schoolbag. Also, it is as small as a usual textbook, an exercise book and so on. They can be made into _ that are as small as a stamp. The students can read the text page by page on the "screen", take notes, or even send e-mails to their teachers. All they have to do is to put the right chip into the e-schoolbag and press the button. It is said that e-schoolbags have been put into use in some Chinese middle schools. In 2012, some students in Shanghai started to use e-schoolbags in the new term. Some say that e-textbooks can be easily broken. Some say it is not good to students' eyes. But only time will tell us all. An e-schoolbag is   _  .
a kind of computer chips a small computer bigger than a textbook much heavier than a usual schoolbag