Question

the stone forest, or shilin in chinese, in chinas yunnan province is a phenomenon of nature that was shaped by geological processes over millions of years. this forest of towering stone pillars wasnt always a landscape of sharp, jagged rocks that resemble trees. originally covered by a vast sea, the area accumulated layers of sediment as shells and coral settled to the ocean floor. under pressure, these layers gradually hardened into limestone, which solidified into massive rock formations when the sea level slowly dropped. as the limestone eroded away, only the harder, more resistant parts of the rock remained, forming the spectacular spires and columns that define the stone forest today.
these limestone deposits were enormous, and some of the stone forests pillars tower over the landscape at nearly one hundred feet in height.
the changing climate shaped ecology as well as geology, as hosts of birds, insects, and plants made their home in the stone forest.
a similar result occurred in the zhejiang province, where volcanic activity created huge towers of weathered basalt.
in the open air, these formations were exposed to powerful, slightly acidic rain, which began to dissolve the porous, stone exterior.

Explanation:

The text is about the formation of the Stone Forest in Yunnan, China. It mentions geological processes like sediment accumulation, limestone formation, and erosion. The option "In the open air, these formations were exposed to powerful, slightly acidic rain, which began to dissolve the porous, stone exterior" fits best as it directly follows the description of limestone solidification and precedes the result of erosion - only harder parts remaining.

Answer:

In the open air, these formations were exposed to powerful, slightly acidic rain, which began to dissolve the porous, stone exterior.