Finding the correct gas-air ratio in producer gas drive engine is difficult as gas composition changes over a run, sometimes drastically.
发现生产者气驱发动机正确的气体，空气比过去困难，运行气体成分的变化，有时会大幅下降。
Now 13, Samantha's grades have slipped drastically and she's obsessed with texting, Facebook and her laptop, sometimes falling asleep in her clothes clutching her phone.
如今13岁的萨曼塔成绩一落千丈，沉迷于发短信、上Facebook、玩手提电脑，有时还抓着电话和衣而睡。
Now 13, Samantha's grades have slipped drastically and she's obsessed with texting, Facebook and her laptop, sometimes falling asleep in her clothes clutching her phone.
如今13岁的萨曼塔成绩一落千丈，沉迷于发短信、上Facebook、玩手提电脑，有时还抓着电话和衣而睡。
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