Shenzhen is a city in South China．It is not very big, but it has attracted people of the whole country and the whole world as well．

    Just about thirty years ago, it was only a small fishing village．There were not many people there． Most of them lived on fishing．Things have greatly changed since the 1980s．It became the first special economic zone(经济特区) in China．

    Because of the opening policy(政策), economic changes between China and the outside world have increased greatly．People can come and go easily between Shenzhen and Hong Kong．Both foreigners and Chinese have built a lot of factories, companies and tourists' attractions there．People all over the country have come to find jobs．Many of them have settled down in this new booming(兴起的) city．

    Shenzhen has been developing so fast that it has become a well－known metropolis(大都市) in China．

已知x²+4y²+kz²=36，且x+y+z的最大值为7，则正数k等于（　　）

一辆汽车在高速公路上行驶，由于遇到紧急情况而刹车，以速度$v\left(t\right)=7-3t+\frac{25}{1+t}$ （t的单位：s，v的单位：m/s）行驶至停止。在此期间汽车继续行驶的距离（单位：m）是（   ）