Hot air balloons have hot air inside them, which is less dense as the air around it. Thus it rises. A cold air balloon would sink because the air would be more dense than the air around it. Also, the balloon’s volume would vary in size depending on the weight of the balloon.
1860-х годах, а кульминацией — распространение поточного производства и поточных линий. В 1860—1870-х годах технологическая революция быстро охватила Западную Европу, США, Российскую империю и Японию.
Probably ion with a positive 1 charge.
Explained briefly
The answer is a ) electromagnetic waves
