That may be difficult to realize, however, because an unknown number of doses is already tied up through contracts between pharmaceutical companies and countries that can afford them.

不过，因为不知多少疫苗已经被买得起的国家和制药厂签订采购合同预订了，这些计划实现起来也许有些困难。

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For infrared surveillance system, it is most difficult that the measurements are only angles not distance, which makes target location of fire control system be difficult to realize.

对于红外预警系统，目标定位的最大难点是观测量只有角度，没有距离，这就使得火控系统的目标导引遇到困难。

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What some teenagers don't realize is how difficult life can be after they get addicted to drugs.

一些青少年没有意识到一旦他们染上毒瘾，生活会变得多么困难。

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They realize it can be difficult to balance college classes and a job during the school year.

VOA: special.2010.06.18

Cost estimates such as the USDA's exclude "any intrinsic benefit that parents realize from child rearing, which would be extremely difficult to monetize, " Prof.

WSJ: Kids Can Be Costly Long After They Turn 18

You realize when you begin reading this that it may be difficult to narrow down to the few people that you really want to look at.

NPR: Airport Security Marked by Hassles