The correct alternative among the options provided is:
High altitude because of low density; high speed to reduce travel times.
Explanation:
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High Altitude:
- Low Density: At cruising altitudes of about 10 km (approximately 33,000 feet), the air density is much lower than at lower altitudes. This reduced density decreases the drag on the aircraft, allowing it to maintain efficient performance. With less atmospheric resistance, aircraft can fly more economically and achieve better fuel efficiency.
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High Speed:
- Reduce Travel Times: Most commercial jets cruise at speeds around Mach 0.8, which is subsonic but close to the speed of sound. At this speed, they can cover large distances in a reasonable amount of time, making air travel more efficient for passengers and cargo transport. The combination of flying at a high altitude and at a high speed optimizes the aircraft’s performance, enabling faster journeys while maintaining fuel efficiency.
Other Options:
- Reduced Weight is not primarily a reason for cruising at high altitudes or high speeds.
- Low Drag and Sufficient Lift: While lower drag at high altitudes helps with efficiency, speed is specifically related to time reduction rather than a direct need for lift.
- Low Pressure: While flying high does involve lower pressure, it is the low density that primarily affects performance. High speeds also don't primarily stem from low pressure, but rather from design choices for efficiency and passenger comfort.
Thus, the main reason for cruising at high altitudes and speeds is due to the reduction in air density at those altitudes, which allows for optimal fuel consumption and travel time.
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