The net force is 270 N
Explanation:
We can solve this problem by using Newton's second law, which states that the net force on an object is equal to the product between its mass and its acceleration:

where
F is the force
m is the mass
a is the acceleration
In this problem, we have
m = 90.0 kg

Substituting, we find the net force on the object:

Learn more about Newton's second law:
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The correct answer is B because isotopes have different numbers of neutrons, and neutrons are located in the nucleus
Hope this helps