13. A nuclear decay chain represents the reason why only fission is more efficient than fusion for nuclear energy
a series of
decays producing sequentially more stable nuclei
b. the only possible type of decay that an isotope can undergo d. the process of nuclei becoming
increasingly energetic as they release radiation

a series of  decays producing sequentially more stable nuclei

Explanation:

A nuclear decay chain represents a series of decays producing sequentially more stable nuclei.

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.

it is b. absorbs thermal energy