Answer:
The function has a maximum in 
The maximum is:

Explanation:
Find the first derivative of the function for the inflection point, then equal to zero and solve for x




Now find the second derivative of the function and evaluate at x = 3.
If
the function has a maximum
If
the function has a minimum

Note that:

the function has a maximum in 
The maximum is:

The answer is d, gravity is the only force acting on the object
Work done = force * distance
work done = 200 * 20
work done = 4000J
30 grams of radioactive isotope have passed.