问题 多选题

奶奶年轻时用煤球煮饭,现在家庭用煤做成蜂窝状,这种变化是为了(  )

A.增大煤与空气的接触面积

B.减少一氧化碳的产生

C.防止温室效应的产生

D.降低煤的着火点

答案

A、“煤球”到“蜂窝煤”增大了煤与氧气的接触面积,能够使煤充分燃烧;故对

B、这种变化是为了促进可燃物燃烧,从而减少产生一氧化碳等有害物质;故对

C、煤充分燃烧生成的是二氧化碳,二氧化碳就是一种温室气体,不能防止温室效应的产生;故错

D、可燃物的着火点是固定的,所以不能降低煤的着火点;故错

故选AB.

问答题
问答题

When workers become more efficient, it’s normally a good thing. But lately, it has acted as a powerful brake on job creation. And the question of whether the recent surge in productivity has run its course is the key to whether job growth is finally poised to take off.

One of the great surprises of the economic downturn that began 27 months ago is this.. Businesses are producing only 3 percent fewer goods and services than they were at the end of 2007, yet Americans are working nearly 10 percent fewer hours because of a mix of layoffs and cutbacks in the workweek.

(46) That means high-level gains in productivity--which in the long run is the key to a higher standard of living but in the short run contributes to sky-high unemployment. So long as employers can squeeze dramatically higher output from every worker, they won’t need to hire again despite the growing economy.

(47) On Friday, the Labor Department will release a closely watched March employment report expected to show the pest job growth in three years, driven by stabilization in the economy and a rebound from February snowstorms.

A p March job-growth number-at a time when the economy is growing at only a middling pace--would suggest that the productivity boom has largely run its course. (48) Regardless, the question of what caused the burst in workers’ efficiency is one of the great unanswered questions of the expansion and has huge stakes for the economy over the coming year.

"It is an episode that we’re going to--we, economists in general--are going to want to understand better and look at for a long time," Federal Reserve Chairman Ben S. Bernanke said at a hearing last week in which he described the productivity gains as "extraordinary" and acknowledged he had not foreseen them.

(49) Businesses have certainly not been investing in new equipment that might enable workers to be more efficient-capital expenditures plummeted during the recession and are rebounding slowly. (50) And the structural shifts occurring in the economy are so profound that one would expect productivity to be lower, rather than higher, as people need new training to work in parts of the economy that are growing, such as exports and the clean-energy sector.

So what’s happening As best as anyone can guess, the crisis that began in 2007 and deepened in 2008 caused both businesses and workers to panic.

(46) That means high-level gains in productivity--which in the long run is the key to a higher standard of living but in the short run contributes to sky-high unemployment.