Once a biological control agent is released, it is difficult to control its speed.
一旦实施了生物控制手段，它的速度就很难控制。
It is difficult to isolate the factors that led to the next development—the emergence of urban settlements.
推动其进一步发展，即城市聚居点出现的诸多因素，是很难孤立出来看待的。
It is difficult to separate legend from truth.
传说与事实很难分辨。
It is difficult to define what makes him so popular.
很难解释清楚什么原因使他如此走红。
It is difficult to assess the full extent of the damage.
损失情况难以全面估计。
It is difficult to keep up with the rapid pace of change.
跟上迅速的变化是很困难的。
It is difficult to make oneself concentrate for long periods.
让自己长时间聚精会神是很困难的。
It is difficult to find work in the present economic climate.
在目前这种经济大气候下很难找到工作。
With so many variables, it is difficult to calculate the cost.
有这么多的可变因素，很难计算出成本。
It is difficult to understand the full import of this statement.
很难理解这份声明中的全部含意。
It is difficult to define the border between love and friendship.
爱情和友情之间的界线难以划清。
It is difficult to measure the success of the campaign at this stage.
在现阶段还难以估量这场运动的成败。
It is difficult to make a comparison with her previous book—they are completely different.
这很难与她以前的书相比—两者是截然不同的。
It is difficult to quantify an exact figure as firms are reluctant to declare their losses.
很难确定一个准确的数字，因为各家公司不愿公布它们的亏损情况。
In large cities, people complain it is difficult to breathe.
在大城市，人们抱怨呼吸困难。
These days it is difficult to imagine a world without books.
如今，很难想象一个没有书的世界。
It is difficult to build a dam in the sea, and the cost is very high.
在海里建水坝十分困难，成本也很高。
It is difficult to figure out a global population of polar bears as much of the range has been poorly studied.
由于大部分有关其生活范围的研究还很少，所以很难统计出全球北极能的数量。
His parents believe that he needs to spend more time on his homework because it is difficult to become a professional sports star.
他的父母认为，他需要花更多的时间在功课上，因为要成为一名职业体育明星是很难的。
Some people complained that it is difficult to deal with wet garbage, as they are asked to remove wet garbage from its bag when dumping.
一些人抱怨说，湿垃圾很难处理，因为按要求在倾倒垃圾时要把湿垃圾从袋子里拿出来。
It is difficult to attract young readers.
很难吸引年轻读者。
It is difficult to sound natural or plausible.
这很难听起来自然或可信。
It is difficult to teach in such a short time.
要在这么短的时间内教学很难。
In fact, it is difficult to buy a toy for a girl that is not pink.
事实上，给一个女孩买一个不是粉红色的玩具是很困难的。
When a city's population grows by 5% a year, it is difficult to keep up.
当一个城市的人口以每年5%的速度增长时，基础设施很难跟得上。
You won't know what he means at first, for it is difficult to understand him.
起初你不知道他是什么意思，因为很难理解他。
It is difficult to train pigeons to escape from a certain situation by pecking.
训练鸽子通过叼啄来脱离某种情境是很困难的。
With such a long interval, it is difficult to determine the age of this fossil.
时间隔得这么久了，很难确定这化石的年代。
Some people believe that it is difficult to protect users' privacy on social network sites.
一些人认为很难在社交网站上保护用户的隐私。
These methods may be useful for a while, but it is difficult to solve this problem completely.
这些方法可能暂时有用，但要完全解决这个问题是很困难的。
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