the world's current population is 7.3 billion and the world population in 1915 was 1.8 billion. The increase over the past century is 7.3 billion - 1.8 billion = 5.5 billion. So, the correct answer would be "more than 5 billion" (option D).
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Answer:
Caves, caverns, sinkholes, funnel-shaped holes, underground rivers and lakes
Explanation:
The karst topography is found at places that are dominated by rocks made out of calcium carbonate, better known as limestone. These rocks react with the water very easily and get dissolved from it very easily. Because of the large scale dissolving, this type of topography has numerous specific features. Some of the most common features in the karst topography are the caves, sinkholes, caverns, funnel-shaped holes, underground rivers, underground lakes etc. All of them are very attractive and understandably cause a lot of interest, thus they are often tourist destinations, but more importantly of big scientific interest. The biggest problem with this topography is that it is generally not stable, and it is very common that the rocks can crumble, which makes them relatively dangerous if visited at certain periods of the year, especially when there is more precipitation.
