31. Which of the following caused the greatest harm to te Aztecs？
A. Lack of food.
B Strong enemies
C. A kind of ill ness
D. Weapons brought from spain
D The head of the Virgin Hyperloop（超级高铁）transport system says the company aims to begin carrying passengers by The Virgin Hyperloop system is based on a technology called magnetic levitation（磁悬浮）。 It uses magnetic fields to lift a vehicle off the ground a little and to push it forward Some high-speed trains also use this technology. But unlike e trains. a p erloop is designed to operate within tubes containing very little air. Electric fc orce moves vehicles, called pods, through the tubes at high speeds. The system offers nearly silent travel and reportedly releases no carbon emissions.
The system’s pods are designed to move at much higher speeds, up to 1， 200 kilometers per hour. It says such a vehicle could complete a trip between New York and Washington, D. C in 30 minutes. That would be two times as fast as a jet flight and four times faster than a high-speed train