Answer:
Variation
Explanation:
This explains variation because theres many different species in the question
Satellites are objects which orbit the planet. The natural satellite of Earth, in this case, is moon
Answer:
Value of acceleration in each case is 
Explanation:
According to newton's law :
...equation 1.
In the given case,
Force by both the children - Friction force .

Putting value of F in equation 1.

Now, if friction force = 20 N.
Therefore, 
Putting value of F and a in equation 1.

Hence , this is the required solution.
Answer:
C. Adding some of the unknown liquid to red litmus paper.
Explanation:
Since the student doesn't know what the liquid is, it is not safe to experiment with it by boiling, cooling, or adding water to it. The safest option would be to add a small bit of the liquid to the litmus paper, which will identify the liquid as neutral, basic, or acidic.