原句重点在a land of opportunity = free of competitors and predators, also full of carbon dioxide and sunlight. 只要选项中没有漏掉这些重点词，就是正确选项。故选项为A
B选项遗漏了competitors and predators， C选项遗漏了光合作用，D选项毫无事实依据，逻辑混乱。
过渡点：chemical changes and a thicker exterior; overcome the conditions. 冠词the特指condition， 则前面句子中需要涉及到difficult condition. 因此只能在C,D中选。而另一关键词chemical changes and a thicker exterior则说明前面句子中应有ancient algae的改变， 综合这两点，只能选择D.
空在两段中
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过渡点either case，这说明这句话之前应该有两个并列的case。而只有第二个插入口前一句“The water is usually naturally occurring groundwater that seeps down along fractures in the rock; less typically , the water is artificially introduced by being pumped down from the surface.”这句话正好讲到两种类型的水，把句子插入之后，也逻辑合理，所以答案是第二个空。