速看！中山大学2020年硕士研究生入学考试初试成绩查询公告发布

大地公司因为业务需要，急需一批电脑，公司董事长张某便将这件事告诉了他的朋友孙某，孙某找到海洋电脑公司，在大地公司不知情的情况下，以大地公司的名义与海洋电脑公司订立了电脑买卖合同，合同金额为80万元，并约定于7月20日交货。合同签订后的第三天，海洋电脑公司向张某发出确认书要求大地公司对该合同加以确认。董事长张某在确认书上表明同意孙某与海洋电脑公司签订的买卖合同，但希望交货日期改为6月12日，并由海洋电脑公司亲自送货上门。海洋电脑公司收到确认书后，表示同意董事长张某的请求。6月11日，大地公司收到了海洋电脑公司的同意书。6月10日，海洋电脑公司将该批电脑交付铁路部门运输，由于泥石流使71台电脑毁损，6月12日，大地公司仅仅收到29台电脑。之后，大地公司以海洋电脑公司违约为由拒绝付款，海洋电脑公司以大地公司不支付价款为由向法院起诉。问：
(1)孙某与海洋电脑公司订立的买卖电脑合同的效力如何?为什么?
(2)在大地公司更改交货日期后，孙某与海洋电脑公司订立的买卖电脑合同的效力如何?为什么?
(3)如何认定大地公司更改交货日期的法律性质?为什么?
(4)大地公司于6月11日收到海洋电脑公司的同意书，会产生何种法律后果?为什么?
(5)71台电脑毁损、灭失的风险应该由谁承担?为什么?

设 ,其中 具有二阶连续偏导数 具有二阶连续导数,求

Kentish Town Road is 8 humdrum high street in north London.It contains pawnbrokers,pound shops,hairdressers and some long-in-the-tooth hardware slores.Unlike Camden Town to the south,full of bars and tattoo parlours,or Hampstead to the west,with its bisLros and boutique clothing shops,little seems to have changed on the street for the past three decades."It's never quite got going,"admiLs Gary McLaren,a local bookseller.Yet the lack of change is odd-and hints at some of the strangeness of London.Kentish Town has excellent Lransport links to ceniral London,and plenty of residenLs prepared to pay good money for thai.Off the high street streich rows of pretty Victorian terraced houses,which sell for as much as 2m a piece.Between 2007 and 2014 property prices in the posLcode area surrounding the main Tube and railway station more than doubled.An influx of French parents,drawn by a school that opened in 2011,is pushing prices even higher.Yet Kentish Town's shops and cafes are almost invariably untrendy and in some cases mouldering.A hair salon,a butcher and a sportswear shop have each been owned by the same men for more than a quarter of a century.Why?One explanation is Lhat,in common with other parts of London,Kentish Town has lots of social housing as well as costly Viclorian terraces.Camden Council,the local authority,is building even more in the borough.This helps cheaper shops survive,suggests Tony Travers of the London School of Economics:council tenants are less likely to drive and so rely more on local outlets.And the sheer volume of car and lorry traffic on the busy high street,which is a main road into the city,might deter shoppers from visiting and swanky businesses from setting up in the area.Demography plays a part,too.Fully 72%of the population of Kentish Town is white,including a good number of Irish residents-higher than the proportion in London as 8 whole,at 60%.Unlike the high streets around Peckham and Brixton in south London,which cater for African shoppers who may travel far to reach them,few specialist shops draw people to Kentish Town."We're not a destinaLion high street,"sighs one local trader.NIMBYs have not always helped.Lots of civic groups are active in the area,campaigning against late licences and the like,says Dan Camer of the Camden New Journal,a newspaper.A local business association is also good at complaining.Partly because of this,a big supermarket has not yet opened on the high street-though Lidl,a discounter,will seL up shop this year."We quite like that il is rough around the edges,"says Michael Williams,a writer and local.Paradoxically,soarinS;house prices in the area might be another brake on change.Wealthy family buyers mean that some houses once split into flats have been tumed back into homes,says Mr Carrier.The result is fewer shoppers on the high street.Wealthy residents are more likely to get their groceries online or drive to bigger stores.And most will go out to the West End rather than a local restaurant.Such"counter-currents"will prevent Kentish Town from gentrifying fully,suggests Gillian Tindall,a local historian.And they affect many other streets in London,too.Lupus Street in Pimlico serves a large council block on one side and white stucco houses on the other.Caledonian Road in Islington,close to rapidly-changing King's Cross,is still fuU of kebab shops.London is a global city,but it is also a collection of villages,cranky and resistant to change.

Gary McLaren admits that

A.few tourists come to visit the town.
B.social housing tenants often shop at local outlets.
C.demography plays a central role in the change.
D.the road remains unchanged for decades.
E.housing price may be an obstacle of change.
F.counter-currents have influenced many streets.
G.property prices near subway and railway station have doubled.