气相色谱法测定水中三氯乙醛时，为什么选用电子捕获检测器？

对噬菌体的描述不正确的是

A．赋予宿主菌生物学性状的遗传物质
B．体积微小，用电子显微镜观察
C．具有细胞结构
D．有严格的宿主特异性
E．可以侵袭细菌

团结、人心和是战胜竞争对手的关键因素。

新航路开辟促进了世界各地物种的交流，下列哪些是从欧洲交流到美洲的物种（）

①玉米

②甘蔗

③马

④骡

A.③④

B.②③④

C.①②

D.①

When Dave was eighteen, he bought a secondhand car for 200 so that he could travel to and from work more (1) than by bus. It worked quite well for a few years, but then it got so old, and it was costing him (2) much in repairs that he decided that he had better (3) it.

He asked among his friends to see if anyone was particularly (4) to buy a cheap car, but they all knew that it was falling to pieces, so (5) of them had any desire to buy it. Dave’s friend Sam saw that he was (6) when they met one evening, and said, "What’s (7) , Dave"

Dave told him, and Sam answered, "Well, what about advertising it in the paper You may (8) more for it that way than the cost of the advertisement!" Thinking that Sam’s (9) was sensible, he put an advertisement in an evening paper, which read "For sale: small car, (10) very little petrol, only two owners. Bargain at 50. " For two days after the advertisement first appeared, there was no (11) . But then on Saturday evening he had an enquiry. A man rang up and said he would like to (12) him about the car. "All right, " Dave said, feeling happy. He asked the man whether ten o’clock the next morning would be (13) or not. "Fine, " the man said, "and I’ll (14) my wife. We intend to go for a ride in it to (15) it. "

The next morning, at a quarter to ten, Dave parked the car in the square outside his front door, (16) to wait there for the people who had (17) his advertisement. Even Dave had to (18) that the car really looked like a wreck. Then, soon after he had got the car as clean (19) it could be, a police car stopped just behind him and a policeman got out. He looked at Dave’s car and then said, "Have you reported this (20) to us yet, sir\

11（）

A. doubt

B. help

C. trouble

D. answer