Read the following paragraph carefully and choose the best topic sentence from the four possible answers that follow the paragraph by writing A, B, C or D.

Since there are more than 80 million current and former smokers in the United States, a widespread CT scanning program of at-risk individuals would De enormously costly. Low-dose CT scans also expose patients to considerably more radiation than standard chest X-rays do, and little is known about how the cumulative risks of years of such scans would balance out with the benefits of the information they provide. Also, more scans would also mean more false positive results, leading to more additional tests and surgical procedures performed on people who do not turn out to have any disease, further complicating the picture.

A、CT scans should be improved in more than one way.

B、CT scans can be very costly in the United States.

C、CT scans may not present clear picture for patients and doctors.

D、There are disadvantages in using CT scans to fight diseases.

Topic sentence : _________________________

Passage 1

For 28 years I drove a New York City taxi. Now, if you were to ask me what I had for breakfast yesterday, I probably couldn’t tell you. But the memory of one fare is so vivid. I’ll remember it all my days.

It was a sunny spring morning. I was cruising down York Avenue looking for a customer. A well-dressed man reached the cab and jumped in. “LaGuardia Airport, please,” he said.

As always, I wondered about my passenger. Was this guy a talker, a mummy, a newspaper reader? After a few moments, he started a conversation. “How do you like driving a cab?”

”I make a living and meet interesting people sometimes. But if I could get a job making $100 a week more, I’d take it—just like you would.”

His reply intrigued me. “I would not change jobs if it meant I had to take a cut of a hundred a week.”

I’d never heard anyone say such a thing. “What do you do?”

“I’m in the neurology department at New York Hospital.”

I’ve always been curious about people, and I’ve tried to learn what I could from them. Many times during long rides, I’d developed a rapport with my passengers—and quite often I’d received very good advice from accountants,lawyers, and plumbers. Maybe it was that this fellow clearly loved his work; maybe it was just the pleasant mood of a spring morning. But I decided to ask for his help.

“Could I ask a big favor of you?” He didn’t answer. “I have a son, 15, a good kid.He’s doing well in school. He wants a job. But a 15-year-old can’t get hired unless his old man knows someone who owns a business, and I don’t.” I paused. “Is there any possibility that you might get him some kind of a summer job—even if he doesn’t get paid?”

He still wasn’t talking, and I was starting to feel foolish. Finally,at the airport, he said, “Well, the medical students have a summer research project. Maybe he could fit in. Have him send me his school record.” He scribbled something and paid me. It was the last time I ever saw him.

That evening, sitting with my family, I pulled the scrap from my pocke.“Robbie,” I announced proudly, ‘"this could be a summer job for you.” He read it outloud:“Fred Pium, N.Y. Hosp.“ ”Is this a joke?” said Robbie.

After I nagged, cajoled, yelled, and finally threatened to cut off his allowance, Robbie sent off his grades the next morning. But gradually the incident was forgotten.

Two weeks later, when I arrived home from work, my son was beaming. He handed me a letter addressed to him.

Robbie got the job. The white lab coat he wore made him feel a lot more important than he really was as he followed Dr. Plum around the hospital, doing minor tasks for him.

The following summer, Robbie worked at the hospital again. He was given more responsibility. As high school graduation neared, Dr. Plum was kind enough to write letters of recommendation for college. Much to our delight, Robbie was accepted at Brown University.

Robbie worked at the hospital for a third summer and gradually developed a love of the medical profession. He applied to medical school, and Dr. Plum again wrote letters.

Robbie was admitted to New York Medical College.Some might call it fate, and I guess it was. But it shows you that big opportunities can come out of ordinary encounters—even something as ordinary as a taxi ride.

What was the narrator’s habit as a taxi driver?

设向量组α1=(3,1,2)T，α2=(2,1,0)T，α3=(1,0,a)T线性无关，则数a的取值应满足________。