练习：
1 .What have scientists found about Saturn?
A They have found a new moon orbiting Saturn.
B They have found methane-based life on Saturn.
C They have found methane-based life on Titan.
D They have found earthlike life on a Saturns moon.
2. What do scientists say about Titan?
A There are life clues there.
B There is acetylene there.
C Water on Titan exists in the form of ice.
D Rivers and lakes there contain life formls.
3. To date，scientists have not yet detected this form of life.(paragraph 5)What does"this form of life" refer to?
A Water-based life.
B Methane-based life.
C Liquid-water-based microorganisms.
D Gas-based life.
4. What can be inferred from what Allen said?
A Scientists have different arguments over whether there is life on Titan.
B Scientists all agree that there is life on Titan.
C Scientists all suggest that a biological explanation is reasonable.
D Scientists all agree that a non-biological chemical reaction is a possible explanation.
5. Which of the following can replace the title of this passage?
A Earthlike Living Beings Found on Titan.
B Finding of One More Moon of Saturn.
C Titan，a New Satellite Found.
D A different Life Form， a Possibility.
答案与题解：
1. C 短文的第一段提供了答案。该段告诉读者，科学家在土星的卫星土卫六（ Titan ）上发现了生命的迹象，这些生命的存在是以甲烷为基础的。
2.A根据第二段"Scientists have reportedly discovered clues showing primitive alien beings arebreathing in Titans dense atmosphere filled with hydrogen."，说明土卫六（Titan）有生命迹象。因此A是答案。
3. B this form of life 指的是土卫六（ Titan ）上以甲烷为生命基础呼吸氢气的生物，即该文讨论的主题。
4.A科学家至今对土卫六是否存在生物有不同的观点和解释。Allen 说要做出土卫六上有生物的结论，首先必须对Titan 上没有生物的观点做出回应；要排除土卫六上可能没有生物的观点，我们还须付出努力。所以只有A是正确选择。
5.D该短文的主题是，科学家在土卫六（ Titan ）上发现了一种与地球生物不同的生命形式的迹象，即以甲烷为生命基础呼吸氢气的生物，但科学家还未能最后验证这一发现。第四段中的一个句子能够揭示该短文的主题："If these signs do turn out to be a sign of life， it would bedoubly exciting because it would represent a second form of life independent from water-based life on Earth."