根据短文内容，选择正确答案
B
    We can not live a modern life without travelling. The fastest way of travelling is by air. With a plane one can travel in one day to places which it takes a month or more to get to a hundred years ago. 
    Travelling by train is slower than by plane, but one can see the country while-he is travelling through. Modern trains have comfortable seats and dining-cars. They make even the longest journey enjoyable.
    Some people prefer to travel by sea when possible. There are large luxury liners(客轮) or river boats. They are not as fast as trains or planes, but travelling by sea is a very pleasant way to spend a holiday.
    Many people like to travel by car. They can make their own timetable. They can travel hundreds of miles a day, just as their wish. They can stop wherever they want to see something interesting or to enjoy a good meal at a good restaurant, or to spend the night at a hotel. That is why people choose travelling by car for pleasant trips, while they usually take trains or planes for business.

如果a₁，a₂，…，a₈为各项都大于零的等差数列，公差d≠0，则（   ）

电脑上使用的鼠标都有一个球形的轮子，在使用过程中与鼠标垫之间的摩擦：（   ）

下列有关克隆技术和胚胎工程的叙述正确的是（   ）

—        I’m very busy now. Could I call you back after work?
— OK. I’ll wait for you.