单项选择题
A．computers are hard to reach B．everyone has their computers locked C．computers are not very secure D．companies ask them to do it
A．it is large B．it is very small C．it takes little time for a computer to read D．it is the size of a short note
A．made by the hackers and put on computers B．not found by the experts C．prepackaged and found on many computers D．hard to find