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 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 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。
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。
It is difficult to conceive of vigorous economic growth without an efficient transport system.
如果没有高效的交通系统，很难想象会有强劲的经济增长。
It is difficult to engrave in marble.
大理石很难雕刻。
It is difficult to obtain reliable evidence.
很难获得可靠的证据。
It is difficult to fight down a desire to sleep.
克服睡意是很困难的。
It is difficult to relate these results with any known cause.
这些结果很难与任何已知的原因联系在一起。
It is difficult to understand his books, much more his lectures.
他写的书很难懂，他的讲演就更难懂了。
It is difficult to define love.
给爱情下定义是很难。
It is difficult to teach a heart.
教会人用心是很难得。
这很难解释。
It is difficult to learn Japanese well.
学好日文很困难。
It is difficult to gauge one's character.
评价一个人的品质是很难的。
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