Before the medical exam, you must fast.
在体检之前,你必须禁食。
You must have seen this kind of fast food chain restaurant.
你一定见过这种快餐连锁餐厅。
And the fourth requirement, though very detailed, does not tell you what the system must do; instead, it specifies how fast the system must be.
第四条需求虽然非常详细,但并没有告诉你系统必须做什么;它只是规定了系统必须有多快。
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