練習(xí)：
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.
答案與題解：
1. C 短文的第一段提供了答案。該段告訴讀者，科學(xué)家在土星的衛(wèi)星土衛(wèi)六（ Titan ）上發(fā)現(xiàn)了生命的跡象，這些生命的存在是以甲烷為基礎(chǔ)的。
2.A根據(jù)第二段"Scientists have reportedly discovered clues showing primitive alien beings arebreathing in Titans dense atmosphere filled with hydrogen."，說(shuō)明土衛(wèi)六（Titan）有生命跡象。因此A是答案。
3. B this form of life 指的是土衛(wèi)六（ Titan ）上以甲烷為生命基礎(chǔ)呼吸氫氣的生物，即該文討論的主題。
4.A科學(xué)家至今對(duì)土衛(wèi)六是否存在生物有不同的觀點(diǎn)和解釋。Allen 說(shuō)要做出土衛(wèi)六上有生物的結(jié)論，首先必須對(duì)Titan 上沒(méi)有生物的觀點(diǎn)做出回應(yīng)；要排除土衛(wèi)六上可能沒(méi)有生物的觀點(diǎn)，我們還須付出努力。所以只有A是正確選擇。
5.D該短文的主題是，科學(xué)家在土衛(wèi)六（ Titan ）上發(fā)現(xiàn)了一種與地球生物不同的生命形式的跡象，即以甲烷為生命基礎(chǔ)呼吸氫氣的生物，但科學(xué)家還未能最后驗(yàn)證這一發(fā)現(xiàn)。第四段中的一個(gè)句子能夠揭示該短文的主題："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."