Question

up, up, and away! hot - air balloons fascinate, amaze, and delight people of all ages. these colorful creations seem to float effortlessly across a clear blue sky. yet they are strong enough to support multiple passengers and equipment. how is that possible? it all comes down to one thing - science. hot - air balloons are based on a single concept: warm air rises and cool air falls. this is because cold air is denser than warm air. density is defined as the ratio of mass to volume; it tells us how many molecules occupy a space at one time. the more molecules there are in a given volume, the denser the substance. the fewer molecules there are, the less dense the substance is. warm air, in which the molecules are farther apart, is less dense than cold air. for liquids and gases, a substance that has greater density sinks to the bottom. hot - air balloons can carry passengers high above the ground. which sentence from the passage best explains how air density impacts hot - air balloons? 1. “hot - air balloons are based on a single concept: warm air rises and cool air falls.” (paragraph 2) 2. “to inflate a balloon, the pilot turns on a burner, which is typically attached to a cylinder of propane gas.” (paragraph 3) 3. “but these unnamed balloons were often lost, making the data they collected useless.” (paragraph 4) 4. “many agencies now use hydrogen - or helium - filled balloons to conduct research in this manner.” (paragraph 5)

Explanation:

The passage explains that hot - air balloons work based on the density difference between warm and cold air. Warm air is less dense than cold air, so it rises. This principle is key to how hot - air balloons float. The sentence "Hot - air balloons are based on a single concept: warm air rises and cool air falls" best explains how air density impacts hot - air balloons.

Answer:

1. "Hot - air balloons are based on a single concept: warm air rises and cool air falls." (Paragraph 2)