以下说法正确的是( ) A.物体做匀速运动,它的机械能一定守恒 B.物体所受合力

题型:选择题

问题:

以下说法正确的是(      )

A.物体做匀速运动,它的机械能一定守恒

B.物体所受合力的功为零,它的机械能一定守恒

C.物体所受的合力不等于零,它的机械能可能守恒

D.物体所受的合力等于零,它的机械能一定守恒

考点:动能定理
题型:选择题

下列说法正确的是[ ]

A.自然界中的所有原子都处于基态

B.同一原子处于激发态时的能量一定高于基态时的能量

C.无论原子种类是否相同,基态原子的能量总是低于激发态原子的能量

D.激发态原子的能量较高,极易失去电子,表现出较强的还原性

题型:选择题

动物在自然界中的作用不包括下列中的(  ) 

A.在维持生态平衡中起着重要作用

B.能够净化空气

C.能帮助植物传粉,传播果实和种子

D.促进了生态系统中的物质循环

题型:选择题

Reading comprehension.

     One evening after dinner, Mr and Mrs. Tisich called a family meeting. "We've had to make a difficult

decision," Mr. Tisich announced. "You see, your mother has been offered a post as co-director of a television

station in Chicago Unfortunately, the station is not here. After thinking long and hard about it,we've concluded

that the right decision is to move to Chicago."

     Marc looked shocked,while his sister Rachel breathlessly started asking when they'd be moving. "It's

surprising,but exciting!" She said. Marc simply said, "We can't go-I can't leave all my friends.I'd rather stay

here and live with Tommy Lyons!"

     The Tisichs hoped that by the time they moved in August, Marc would grow more accustomed to the idea

of leaving. However, he showed no signs of accepting the news, refusing to pack his belongings.

     When the morning of the move arrived, Marc was nowhere to be found. His parents called Tommy Lyons's

house, but Mrs. Lyons said she hadn't seen Marc. Mrs. Tisich became increasingly concerned, while her

husband felt angry with their son for behaving so irresponsibly.

     What they didn't know was that Marc had started walking over to Tommy's house,with a faint idea of

hiding in the Lyons's attic (阁楼) for a few days. But something happened on the way as Marc walked past all

the familiar landscape of the neighborhood: the fence that he and his mother painted, the tree that he and his

sister used to climb, the park where he and his father often took evening walks together. How much would

these mean without his family, who make them special in the first place? Marc didn't take the time to answer

that question but instead hurried back to his house, wondering if there were any moving cartons (纸板箱) the

right size to hold his record collection.

1. The conflict in this story was caused by _____.

A. Marc and Rachel's different tempers

B. a quarrel between Tommy Lyons and Marc

C. Marc's disagreement with his parents about their move

D. Mr.and Mrs. Tisich's remark of Marc's irresponsibility

2. Marc and Rachel's reactions to the move were similar in the way that both were _____.

A. surprised

B. angry and upset

C. anxious for more details

D. worried about packing

3. The reason for Marc's going home was that _____.

A. he did not want to be left behind

B. he realized his family was essential to him

C. he hoped to reach an agreement with his parents

D. he wished to be a more responsible person

4. What would most likely happen next?

A. Marc would bring his records over to the Lyons's house.

B. Mr. and Mrs. Tisich would call the police.

C. Marc would join his family for house moving.

D. Mr.and Mrs. Tisich would start searching for Marc.

题型:选择题

肺腺癌()

A.中央型多见

B.周围型多见

C.弥漫型多见

D.发生于Ⅰ型肺泡上皮细胞

E.发生于胸膜

题型:选择题

猪流感是由流型性感冒病毒和猪嗜血杆菌或()协同作用而引起的。

A.大肠杆菌

B.多杀性巴氏杆菌

C.链球菌

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