单选题 The successful spread of a virus depends on how long it can (74) unnoticed, before its presence is made known by the activation of side-effects. For example, many viruses (75) their code to avoid recognition.

【正确答案】 B
【答案解析】[解析] 译文：计算机病毒能否成功地传播依赖于它是否能在不被发觉的时候不断复制。
【正确答案】 D
【答案解析】[解析] 译文：例如，许多病毒通过加密自身代码的方式避免被发现和检测。