关系的数据操纵语言按照表达式查询方式可分为两大类：关系代数和 【7】 。

Want a glance of the future of health care Take a look at the way the various networks of people about patient care are being connected to one another, and how this new connectivity is being exploited to deliver medicine to the patient—no matter where he or she may be.
Online doctors offering advice based on normal symptoms are the most obvious example. Increasingly, however, remote diagnosis will be based on real physiological data from the actual patient. A group from the University of Kentucky has shown that by using personal data assistance plus a mobile phone, it is perfectly practical to send a patient’s important signs over the telephone. With this kind of equipment, the cry asking whether there was a doctor in the house could well be a thing of the past.
Other medical technology groups are working on applying telemedicine to rural (countryside) care. And at least one team wants to use telemedicine as a tool for disaster need—especially after earthquakes. On the whole, the trend is towards providing global access to medical data and experts’ opinions.
But there is one problem. Bandwidth is the limiting factor for sending complex medical pictures around the world—CT photos being one of the biggest bandwidth users. Communication satellites may be able to deal with the short-term needs during disasters such as earthquakes or wars. But medicine is looking towards both the second-generation Internet and third-generation mobile phones for the future of remote medical service.
Doctors have met to discuss computer-based tools for medical diagnosis, training and telemedicine. With the falling price of broadband communications, the new technologies should start a new time when telemedicine and the sharing of medical information, experts’ opinions and diagnosis are common.

Which of the following statements is true according to the text

A. Patients don’t need doctors in hospitals any more.
B. It is impossible to send a patient’s signs over the telephone.
C. Many teams use telemedicine dealing with disasters now.
D. Broadband communications will become cheaper in the future.