按要求改写下面句子。

1. Help was often given to him by us. (改为主动语态) 

    We often              him            .

2. Maths is more difficult than any other subject. (改为同义句) 

    Maths is the              difficult             all the subjects.

3. I have written to him for a long time. (改为同义句) 

    He ______ ______ ______ me for a long time.

4. I started to study English two years ago. (改为同义句)

    I                         English for two years.

5. We have an English test once a month. (对划线部分提问) 

                            do you have an English test?

企业确定客户服务水平与控制总成本的基本思想应该是( )。

A．在确定客户服务水平的前提下，使成本最小化

B．在企业可承担的成本前提下，确定可达到的服务水平

C．在追求最高的客户服务水平前提下，确定成本

D．客户服务水平和成本控制是独立的两件事