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插入句子題

空都在一段中

過渡點：chemical changes and a thicker exterior; overcome the conditions. 冠詞the特指condition， 則前面句子中需要涉及到difficult condition. 因此只能在C,D中選。而另一關鍵詞chemical changes and a thicker exterior則說明前面句子中應有ancient algae的改變， 綜合這兩點，只能選擇D.  

空在两段中

過渡點either case，這說明這句話之前應該有兩個並列的case。而只有第二個插入口前一句“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.”這句話正好講到兩種類型的水，把句子插入之後，也邏輯合理，所以答案是第二個空。