单项选择题
The successful spread of a virus depends on how long it can （） unnoticed, before its presence is made known by the activation of side-effects. For example, many viruses （） their code to avoid recognition.
The successful spread of a virus depends on how long it can （） unnoticed, before its presence is made known by the activation of side-effects.
A．move
B．replicate
C．backup
D．die out