Question

complete the text with the detail that best develops the main topic.
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, this 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.
the changing climate shaped ecology as well as geology, as hosts of birds, insects, and plants made their home in the stone forest.
in the open air, these formations were exposed to powerful, slightly acidic rain, which began to dissolve the porous, stone exterior.
a similar result occurred in the zhejiang province, where volcanic activity created huge towers of weathered basalt.
these limestone deposits were enormous, and some of the stone forests pillars tower over the landscape at nearly one hundred feet in height.

Explanation:

The text is about the geological formation of the Stone Forest. The sentence "In the open air, these formations were exposed to powerful, slightly acidic rain, which began to dissolve the porous, stone exterior" best develops the main topic as it continues the geological process description (after limestone formation) by explaining how the limestone eroded (due to rain) to form the current Stone Forest structures. The other options either shift focus (like the one about ecology) or introduce a different location (Zhejiang) or just state a fact about pillar height (which is less about the formation process compared to the rain - erosion explanation).

Answer:

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