Columnar joints form when a. a lava flow erupts under deep water b. during the final stages of cooling, a lava flow contracts, forming tall polygonal fractures c. a lava flow encounters groundwater, which causes the flow to rapidly fracture d. a lava flow cools very rapidly, causing the flow to shatter in a chaotic pattern

b. during the final stages of cooling, a lava flow contracts, forming tall polygonal fractures

Explanation:

The columnar joints are very interesting and attractive for the eye geological features. They are not very common feature which makes them even more interesting. Some of them have been covered by sediments so they are hidden below the surface, but some have remained above the surface, sticking out as the erosion acts much slower on them than the surrounding area. They form when the lava cools off very quickly at its final stages, thus starting to contract, and instead of being smooth, they become polygonal fractures that are very tall.

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.