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.