Taking out travel insurance may seem an unnecessary expense, but it is a must.
旅游保险费用看起来似乎是没必要的，但却是必不可少的开支。
Perhaps that is because computers are less visibly polluting, or their use is not deemed, like air travel, to be frivolous and unnecessary.
或许，这是因为电脑的污染更加不易被察觉，或者其之使用不像航空飞行一样被认为是不重要且不必要的。
Tourism spawns unnecessary litter, when you travel you must think about the environmental impact.
旅游业产生了不必要的垃圾，要旅游同时一定要考虑到环保。
Perhaps that is because computers are less visibly polluting, or their use is not deemed, like air travel, tobe frivolous and unnecessary.
或许，这是因为电脑的污染更加不易被察觉，或者其之使用不像航空飞行一样被认为是不重要且不必要的。
The consumed CPU time of the multiple migration is not too much, because multiple travel-time calculations for the different percentages of the velocity model are unnecessary.
由于无需对不同百分比速度模型重复计算旅行时，多次叠前时间偏移所耗费CPU时间并不多。
The height of return travel of pressing board can be set freely, which reduces unnecessary idle travel.
压板回程高度可任意设定，减少了不必要的空行程。
Other decisions are far more complex, such as deciding where to go to find food or water during a drought or choosing a travel route that avoids unnecessary danger.
有些决定更加复杂，例如，在干旱时候应该到哪里去找食物和水源或者选择相对比较安全的路径。
Other decisions are far more complex, such as deciding where to go to find food or water during a drought or choosing a travel route that avoids unnecessary danger.
有些决定更加复杂，例如，在干旱时候应该到哪里去找食物和水源或者选择相对比较安全的路径。
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