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{{*HTML*}}考虑二元函数f(x，y)的下面4条性质：①f(x，y)在点(x0，y0)处连续； ②f(x，y)在点(x0，y0)处的两个偏导数连续；③f(x，y)在点(x0，y0)处可微； ④f(x，y)在点(x0，y0)处的两个偏导数存在．若用“P→Q”表示可由性质P退出性质Q，则有 ( )
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根据“给定资料3—5”，分析青少年暴力事件频发的原因。 要求：观点明确，分析合理，语言简练。不超过300字。
In agriculture a relevant aspect is the better adaptation of plants to specific environmental conditions that can be gained using genetic engineering techniques: this includes the better adaptation of crop plants to the changing environmental conditions, including climate change, increased UV radiation, changed rainfall patterns. Plants may be able to be more resistant to drought, flooding, salinity or sensitivity to heavy metals, so that they can be grown in areas of the earth currently beyond the tolerance range of species, or even those areas unable to be used for agriculture at all. Furthermore, other environments could be considered: tolerance to low temperature is also important, and, for example, an antifreeze gene from an arctic fish has been transferred to soybean, with the goal of creating plants tolerant to low temperature. Another aspect is related to chemical problems connected with plants growth: aluminum toxicity is a problem in low pH soils, where it may reduce plant growth. By making plants tolerant, they could grow better in such soils. Genetic modifications could lead to an increased disease and pest resistance and to the production of new cultivars with improved pest and disease resistance to promote more environmentally acceptable alternatives for food production. The genetic manipulation could lead finally to the utilization of new raw materials: traditional foods often involve consumption of only one or several parts of a plant, for example fruits, leaves, roots, or stems. Plants grown in one culture for roots may be eaten for their leaves in another, such as beetroot. Some plants may be eaten by humans in one culture, such as the plant rape in Japan, whereas it is used for rapeseed oil production in Europe.[Key Words]salinity n. 盐度；盐分；盐性 cultivar n. 栽培品种beetroot n. 甜菜根 rape n. 油菜
{{*HTML*}}设函数z=(1+ey)cos x—yey，则函数z=f(x，y) ( )
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