题目内容：
阅读下列短文，从每题所给的四个选项(A、B、C和D)中，选出最佳答案You can use your Business Telecard International at any card phone in the United Kingdom. Here is some information about making international phone calls.
You can now phone almost any country in the world, although in some cases you can only call big cities. When you cannot make direct dialing calls, you can ask the international operator to help you. This is more expensive and takes more time, but it may be helpful if you want to speak to a particular person and no one else, in this case you should ask for a “person-to-person” call. Even more expensive is a reverse charge(对方付款)call where the person who receives the call pays.
If the international line is busy, you can reserve(留下)a call, explain the number you want and the operator will call you back when the line is free.
You can save money by calling outside office hours, for example, early in the morning, late at night, and on Sundays. Remember that the time may be different in the country that you are calling. International time is based on GMT (Greenwich Mean Time). London is on GMT and Moscow, for example, is 3 hours ahead. There is one problem: change to DST (Daylight Saving Time) for the summer. In the UK, clocks are put forward one hour in spring and put back in autumn, and so London is actually one hour ahead of GMT in the summer.
根据材料回答题
What is the most expensive type of call? A．Direct dial.
B．Through the operator.
C．Reverse charge call.
D．Calling outside office hours.
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