What is a controlled experiment?
a. two variables are changed at a time
b. all variables are kept constant
c. none of the variables are changed
d. one variable is changed at a time

d

Explanation:

Only one is changed being the independent variable. Adjusted to see the effects of the system being studied.

that tool is called an altimeter which measures your altitude which can be used to tell which atmosphere you are currently in, including the stratosphere.

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when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.