根据句中所给汉语，完成下列句子。（每空一词，每空0.5分，共10分）
小题1:我们必须找到方法以阻止有毒气体污染空气.
Ways must be found to ________ poisonous gas ________ ________ _________.
小题2:这对夫妇靠微薄的收入养大了孩子，现在轮到他们的孩子来照顾年迈的父母了。
The couple brought up their children on a small income. Now _______ _______ _______
________, they should look after their elder parents.
小题3:火星上有没有生命还不确定。
_______ is uncertain ________ ________ ________ life on Mars.
小题4:既然你已经拿到了驾照，为什么不自己开车呢?
_______ _____ you have got your license, _______ ______ drive the car yourself?
小题5:我正在街上徘徊，这时我突然看见了一家裁缝店。
I was wandering through the street ________ I _________ ________ ________ a tailor’s shop.
小题1: prevent/stop/keep from polluting atmosphere/air
小题2: in their children’s turn
小题3:It whether there is
小题4:Now that why not
小题5:when caught sight of
题目分析：
小题1:prevent/stop/keep from polluting atmosphere/air
固定搭配prevent/stop/keep sb from doing sth阻止某人做某事。
小题2: in their children’s turn
固定搭配in one’s turn轮到某人。
小题3: It whether there is
本句中的It是形式主语，真正的主语是后面的whether there is life on Mars。
小题4: Now that why not
连词now that既然，注意其中的that可以省略，“why not…?”是一个表示建议的句型。
小题5:when caught sight of
本句考查了固定词组catch sight of看见…
点评：做这类题，不但要选择正确意义的词、词组及句型，还要根据具体语法和语境，选择适当的时态、语态及使用正确的词形。