Hybrid computers are used to control air traffic and radar of national defence.

Explanation:
Hybrid computers are a combination of digital and analogue computers, which makes them suitable for applications that require both processing power and real-time control. The operations of air traffic control and radar systems require the ability to process large amounts of data quickly and accurately, while also providing real-time control and monitoring.

Advantages of Hybrid Computers:
1. Speed and Accuracy: Hybrid computers can process data quickly and accurately, making them ideal for air traffic control and radar systems where real-time monitoring and decision-making are crucial.

2. Real-time Control: Hybrid computers can handle real-time control tasks effectively, allowing them to control and monitor air traffic and radar systems in real-time.

3. Processing Power: Hybrid computers have the processing power to handle large amounts of data, which is essential for air traffic control and radar systems that generate vast amounts of information.

4. Flexibility: Hybrid computers can be easily customized and adapted to meet the specific requirements of air traffic control and radar systems, making them a versatile choice for these applications.

5. Reliability: Hybrid computers are known for their reliability, which is crucial for critical applications like air traffic control and national defense.

6. Cost-effective: Hybrid computers offer a cost-effective solution for air traffic control and radar systems, as they combine the advantages of both digital and analogue computers without the need for separate systems.

In conclusion, hybrid computers are used to control air traffic and radar of national defense due to their ability to handle real-time control tasks, process large amounts of data quickly and accurately, and provide a cost-effective solution for these critical applications.