【托福阅读题型：Planets in Our Solar System句子插入题】托福阅读中句子插入题是重要题型之一。根据托福阅读题型不同来进行专项练习，是短期提高托福阅读能力的有效方法。同学们需要多加练习哦!

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　　托福阅读句子插入题：Planets in Our Solar System

　　【Paragraph 4】Other dimensions along which the two groups differ markedly are density and composition. The　densities of the terrestrial planets average about 5 times the density of water, whereas the Jovian planets have densities　that average only 1.5 times the density of water. One of the outer planets, Saturn, has a density of only 0.7 that of water,　which means that Saturn would float in water. Variations in the composition of the planets are largely responsible for the　density differences. ■The substances that make up both groups of planets are divided into three groups—gases, rocks,　and ices—based on their melting points. ■The terrestrial planets are mostly rocks: dense rocky and metallic material,　with minor amounts of gases. ■The Jovian planets, on the other hand, contain a large percentage of the gases hydrogen　and helium, with varying amounts of ices: mostly water, ammonia, and methane ices. ■(Planets in Our Solar System)

　　Look at the four squares [■] that indicate where the following sentence could be added to the passage.

　　This explains their relatively low densities.

　　Where would the sentence best fit?

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