U-235, the isotope of uranium commonly utilized in nuclear power plants, is

a. the most common of the naturally occurring isotopes of uranium
b. heavier than the other well-known isotope of uranium
c. rare even in uranium oxide deposits, and must be enriched before use as fuel
d. only found in granite plutons

Option C.

Explanation:

Natural uranium is composed by 99.27% of U-238, 0.72% of U-235 and 0.005% of U-234.

Of the isotopes of natural uranium, the U-235 is the most commonly utilized in nuclear power plants, but since it is only 0.72% of natural uranium it needs to be enriched before being used as fuel to sustain a nuclear fission reaction in nuclear power plants.    

The U-238 is the most common and the heaviest of the three naturally occurring isotopes of uranium.

So, from the above the correct option is the C.    

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C

Explanation:

Uranium is made up three isotopes which are U-234, U-235 and U-238. Among the three isotopes U-238 is the naturally occurring and U-235 is the least occurring.

U-235 is used as a fuel in nuclear reactors and also used in nuclear weapons as explosives. It is a very rare isotope of uranium and present in amounts of only 0.7% of naturally occurring uranium, hence its most treasured among the isotopes.

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.