下列属于数据库管理员的职能有哪些（）。 ["身份验证","资源限制","加密","策略和标记管理","客体重用"]
成功的企业，应该把员工犯错看成企业的一种（）。 ["损失","收益","成本","存款"]
信用社应建立农户信用等级评定小组负责农户信用等级的评定，评定小组的设立应以（）为单位。 ["社","村","村民小组","镇"]
关于赠送礼物，下列说法正确的有（）。 ["不要选择易使异性产生误解的物品","选择探望病人的礼物，应更多地注重精神效应","对外宾送礼，以民族特色为佳","礼物的轻重选择以对方能愉快接受为尺度","选择回赠的礼品通常选择与对方所赠礼品价格大致相同的物品"]
Dirt should not be allowed to contaminate a grease lubricant because the （）. ["dirt will cause corrosion of bearing","bearings will leak oil excessively","dirt is very abrasive when mixed with grease","grease will become inflammable"]
什么是希尔反应？
