责任感和可靠性是必需的素质，友善和外向的性格也必不可少。
Responsibility and reliability are necessary qualifications, as well as a friendly and outgoing personality.
很不幸，在日常生活中疼痛虽然令人很不悦但却是必不可少的。疼痛会警告人们那些可能会伤害到自己的事物来保护他们。 （根据原文的意思疼痛是生活的必需品的而不是避免的，也就是说疼痛本来是对人有很多好处的，你的翻译意思上有点不对。）
PAIN, unfortunately, is a horrible necessity of life. It protects people by alerting them to things that might injure them.
Control - flag：这指定任何特定模块是必不可少的、必需的、可选的还是足够的，从而定义模块的行为。
Control-flag: This defines the behavior of stacking modules by specifying whether any particular module is requisite, required, optional, or sufficient.
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