小明测量植物油密度时，按下列顺序进行了操作：
①用天平测出空烧杯的质量m1；
②向烧杯中倒入适量植物油，测出烧杯与植物油的总质量m2；
③将烧杯中植物油倒入量筒中，测出植物油的体积V；
④计算出植物油的密度ρ。
（1）下表是小明根据上述实验操作，设计的记录数据表格。请根据下图将表格中的数据填写完整。
（2）小明按上述实验操作得到的密度值比真实值偏________。请你在小明实验的基础上，设计一个误差更小的实验方案并写出计算密度的最后表达式。
（1）
（2）大；
①向烧杯中倒入适量植物油，测出烧杯与植物油的总质量m1′；
②将烧杯中一部分植物油倒入量筒中，测出植物油的体积V′；
③用天平测出烧杯和剩余植物油的质量m2′；
④计算出植物油的密度ρ'= （m1′－m2′）/ V′ 。（合理即可）
Passage Three
| Immigrants are consumers as well as producers, so they create jobs as well as taking them. And the work they do need not be at the expense of native workers. Immigrants often hold jobs that natives are unwilling to accept at any feasible wage. Also, immigrants sometimes help to keep industries viable (能存活的) that would otherwise disappear altogether, causing employment to fall. This was the conclusion of a study of the Los Angeles garment industry in the 1970s and 1980s. And when immigrants working for low wages do put downward pressure on natives’ wages, they may raise the (real) wages of natives in general by keeping prices lower than they otherwise would be. In theory, then, the net effect of immigration on native wages is uncertain. Unfortunately, most of the empirical (经验主义的) research on whether immigrants make natives worse off in practice is also inconclusive except the effect, one way or the other, seems small. Most of this research has been done in America: if there were any marked influence on wages, that is where you would expect to find it, given the scale of immigration and the tendency of the newcomers to concentrate in certain areas. But most studies have compared wages and employment in areas with many immigrants to wages and employment in areas with few. For instance, one examined the impact of sudden and notorious inflow of refugees to Miami from the Cuban port of Mariel in 1980. Within the space of a few months, 125 000 people had arrived, increasing Miami’s labor force by 7%. Yet the study concluded that wages and employment among the city’s natives, including the unskilled, were virtually unaffected. Another study examined the effect of immigration on wages and employment of those at the bottom of the jobs ladder—unskilled blacks and Hispanics. It found that a doubling of the rate of immigration had no detectable effect on natives. The most recent work, admittedly, has tended to question these findings. Using more detailed statistics and more sophisticated methods than the earlier studies, this work has tended to find that immigrants’ wages take longer to rise to the level of the natives’ wages than has been supposed. This implies a more persistent downward pressure on the host economy’s labor market. Typically these studies find that immigration does depress unskilled natives’ wages to a small extent. But nearly all economists would agree that the effects of immigration are insignificant in relation to other influences. |
It is certain, according to empirical research, that ______.
A．immigration has a positive effect on native wages
B．immigration makes unskilled blacks and Hispanics worse off
C．immigration has only a small effect on native wages
D．immigration has a marked influence on natives in areas with dense immigrant population