What is the force that can cause two pieces of iron to attract each other?
a.
gravitational force
b.
magnetic force
c.
elastic force
d.
electrostatic force

B. Magnetic Force

Explanation:

two pieces of irons cannot attract each other unless at least one of them is magnetize. That force is called magnetism.

Is it A ?  Yes !  Two pieces of iron DO always attract each other with equal and opposite gravitational forces, although the forces are so small that we never notice them.

Is it B ?  Yes, it could be.  If at least one of the pieces of iron is magnetized, then they can attract each other with equal and opposite magnetic forces.

Is it C ?  Yes, it could be.  But the pieces of iron would have to be joined by a stretched rubber band.  Then they would attract each other with equal and opposite elastic forces.

Is it D ?  Yes, it could be.  If one of the pieces of iron carries a net electric charge, then they attract each other with equal and opposite electrostatic forces.  

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.