单项选择题
A．computers have been widely used by Americans B．computers can solve complex problems C．computers are domestic robots D．computers are new machines
A．sent by telegraph B．sent by letter C．passed from one person to another D．sent by telephone
A．they cover major points of the book in detail B．they may help reader to better understand the book C．they can take readers a lot of time to read every part of the book D．they may help the readers find the important parts of the book