下列实体的联系中，属于多对多联系的是( )。A．学生与课程 B．学校与校长 C．住

某生物兴趣小组准备“探究草履虫对外界刺激作出的反应”，假如你是其中一员，请完善下列内容．

提出问题：草履虫能否对外界的刺激作出反应？

作出假设：______．

设计实验：

①在载玻片的两端分别滴一滴______培养液．

②用解剖针把两滴培养液连通起来．

③在培养液一侧放入少许______，用放大镜观察草履虫有什么反应．

实验现象：草履虫会______．

得出结论：______．

中国甲公司于5月9日发商务电传至加拿大乙公司，该电传称：“可供白糖1500公吨，每公吨500美元，CFR温哥华，10月装船，不可撤销信用证付款，本月内答复有效。”乙公司于6月9日回电：“你方5月9日报盘我接受，除提供通常单据外，需提供卫生检验证明。”甲公司未予答复。甲公司与乙公司之间的合同关系是否成立（）

A.乙公司的回电在实质上变更了要约的条件，因此合同不成立 

B．乙公司的回电未在实质上变更要约的条件，因此合同成立 

C．乙公司未在要约的有效时间内作出承诺，该逾期承诺原则上无效 

D．由于乙公司提出新条件，甲公司未予答复，因此合同不成立

The United States counts its population every ten years, and each census reveals that the racial and ethnic mix is changing dramatically, so by the year 2050, the "average" American will not be descended from (1) , but the majority of U. S. residents will trace (2) ancestry to Africa, Asia, the Hispanic world, the Pacific Islands, or the Middle East. Once the United States was a microcosm of European nationalities, (3) the United States is a microcosm of the world. The United States is (4) considered a "melting pot" society by many of its residents. (5) , many people prefer the term "salad bowl." They use this (6) to describe American society. American society will soon be (7) nonwhite. "Melting pot" implies that the different ethnic groups blend together (8) one homogeneous (9) , "salad bowl" (10) that nationalities, like the ingredients in a mixed green salad, retain their cultural (11) . Earlier generations of (12) believed they had to learn English quickly not only to survive but also for (13) .
Now, many immigrant groups do not (14) the same need. Because there are many places in America (15) you can work, shop, get medical care, marry, divorce and die without knowing English. (16) , Chinatown in San Francisco and New York. Also, Los Angeles has many Vietnamese immigrants and immigrants from Mexico. (17) , many immigrant groups want their children to know their (18) culture. Many Hispanics, for instance, want their children to learn (19) English and Spanish and study the Spanish language in school. They are fighting for the (20) to bilingual education in many communities. In many communities they are in the majority.

各种业务时钟必须与传输时钟保持一致。

浇注式沥青混合料的摊铺使用浇注式专用摊铺机。该摊铺机主要由三部分组成：自行牵引系统、（）以及前置的布料系统。

A.振动压实系统

B.自动化系统

C.加热系统

D.混合料摊铺系统