Designing a cover specifically for an e-book is rare: Most e-book covers are digital images of their print namesakes.
专门为一本电子书设计封面也很少见，大部分电子书的封面都是同名实体书封面的电子版。
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Like their biological namesakes, computer viruses replicate by attaching themselves to a host (a program, or a computer) and coopting the host's resources to copy themselves.
Viewers can add comments, keywords and identify people in images in the three sets - Launch/Takeoff, Building NASA, and NASA Center Namesakes, which records historical figures.