人体细胞要获得氧气，排出二氧化碳，必须经过如下①②③等过程来完成

在这些过程中，发生气体交换的有（    ）

下列运算中，正确的是（　　）

    It's always been a dream for Alex to take a trip with her mother, but it was a death in the family that1made them both realize how precious life really is.

    “After losing my grandma, I2that neither I nor my mum will3be here, and that I can't4the right time to come,” the 27-year-old girl told The Huffington post.

    “After a week of5, my amazing mum and I are off to travel around the United States. We will be travelling from New York to Los Angeles interviewing6who are changing the world for the better and7the dream I had for a very long time: to travel together with my mother. I have taken a month off work and we are on our way.8are waiting!” Alex said.

    So in August，Alex and her 63-year-old retired mother，Halina, traveled to the United States for a three-week 9that took them from coast to coast.

    Alex is the10of Looking For Heroes, a social media project, in which she photographs many of the interesting people she meets with,11unique tidbits (花边新闻) of their life stories at the same time. The 12mother and daughter traveled across the nation, visiting cities13Knoxville, Tennessee, New York City, Los Angeles and Washington. Along the way they took photos 14 famous backdrops (背景) in each city, making some15memories.

    Meanwhile, Alex16much from her mother. Her mother often said “My  17to young people would be to learn new things, and what you have learned will be your biggest 18in life. With age you stop19about things that don't matter. With your priorities (优先) changing, 20and health become the most important.

    Every day, a homeowner in Pennsylvania is reminded of a mistake he made 13 years ago. At around 8 in the evening, his peaceful house comes alive with the noise of an alarm clock he dropped into the wall over ten years ago. It's been stuck in the wall ever since.

    13 years ago, Jerry Lynn wanted to make a hole through his living room wall, so he could pass the television cable (电缆)through it. He didn't want to destroy any pipes in the wall, so he came up with a good idea to make sure that he was drilling(钻孔) in an empty place. He took his alarm clock, tied it to a string(线) and set it to go off in 10 minutes. Then he lowered it into the wall, and waited for it to ring. His goal was to drill a hole in a safe place near the place where the ringing was coming from, making sure that there was nothing to pass through there. It all went smoothly until the alarm clock fell into the wall.

The man didn't get back the alarm clock, but thought it would probably run out of battery(电池)in several months, so he wasn't too worried about it. But 13 years later, the alarm clock still rings every day.

    As you can imagine, Jerry and his wife have gotten used to the ringing living room wall, but it can still surprise  guests. So Jerry decides to pull down that wall, just to stop that boring thing, though after putting up with it for 13 years, he think it will be a pity. He says, “I've never herd of a clock battery lasting that long and my friends think it may be interesting to see how long it will keep ringing for.”