Chemicals that are present high up in the atmosphere damage the material that the balloon is made of.
The material that the balloon is made of cannot be stretched any farther.
The balloon catches fire from the Sun s rays when it reaches a certain altitude.
The pressure level inside the balloon increases steadily as the balloon ascends.

Heat the helium  that is inside the balloon.
Slowly release helium from the balloon as it ascends to maintain a constant volume in the balloon.

Areas where cool, dry air meets warm, moist air.
Areas of land near large bodies of water.

Hot air rises.
Flames are pulled by gravity.
There is less oxygen at the bottom of a slope than there is at the top of a slope.
Vegetation is drier at the bottom of a slope than it is at the top of a slope.

The pesticide was removed from the environment by natural processes.
The introduction of natural predators rendered the pesticide ineffective.
The pesticide only killed gypsy moths that were in the larval stage.
The pesticide did not penetrate gypsy moth pupa cocoons.

It reduced the gypsy moth population in year 2.
It was more effective than the pesticide in reducing the gypsy moth population growth rate in year 2.
Initially it was more effective than the pesticide , but it eventually lost its effectiveness.
Initially it was less effective than the pesticide , but it outperformed the pesticide by year 3.

Birds that migrated individually or in small groups were less likely to survive and have offspring.
Birds that migrated individually or in small groups were more likely to find adequate food.
Flying in large groups allowed other bird species to join the migration.
Flying in large groups allowed each bird to have a better chance of finding a nesting site.

The meteoroid is pulled in by the rotation of Earth.
The meteoroid is pushed by the light of the Sun.
The meteoroid is attracted to the mass of Earth.
The meteoroid is repelled by the vacuum of space.

Increasing the rate of water flow through the tanks.
Increasing the amount of nutrients added to the first tank.
Using filters that allow larger organisms to move between the tanks.
Using the wastes produced by the organisms to make fuel to run the water pumps.