一单色光垂直照射在空气劈尖上，当劈尖的劈角增大时，各级干涉条纹将有下列中的何种变化？

某种商品在两个月内降价两次，现在该商品每件的价格比两个月前下降了36％，问

平均每月降价百分之几？设平均每月降价的百分率为x，则可列方程为           .

下列不属于应收账款管理成本的有( )。

A．无法收回应收账款而发生的费用

B．对客户的资信调查费用

C．催收应收账款而发生的费用

D．因投资应收账款而丧失的利息费用

——Look at your pale face. You ______ a sleepless night.

——You bet. The work site next to the hotel was so noisy.

A．must have had

B．might have

C．should have had

D．cannot have had

首字母填空   (10分)

Now we live in the “computer age”. People like scientists, teachers, writers and even students use computers to do much w______. But more than 30 years ago, c________ couldn’t do much. They were very big and expensive. Quite few people were interested in them and knew how to u________ them. Today computers are smaller and cheaper. And so many people like to use them b_______ they can do a lot of work. Some even use them at home. Computers become i______ because they can work faster and make fewer mistakes. Computers can help people d______ a lot of work. Writers use them to write, students use them to s______ and children use them to play computer games. Computers can r_______ what you put into them. They are very useful and h________. They are our good f_______. We love them.

如果某一结点要进行数据发送，在CSMA/CD控制方法中，则必须( )。

A．可立即发送

B．等到总线空闲

C．等到空令牌

D．等发送时间到