为了生儿育女，鱼儿也不容易

For fish like the porthole livebearer (Poeciliopsis gracilis, below), pregnancy might be less of a drag if they take it slow. 

对于若花鳉鱼来说（下图），假如怀孕时它们能够游得慢些可能负担就不会那么大。

To find out how pregnancy might affect a fish’s swimming speed—and maybe its ability to escape predators—researchers made 3D-printed models of expectant livebearers in their lab, whose ballooning bellies increased their size by as much as 43% during pregnancy. 

为了研究怀孕可能会对鱼的游速造成什么样的影响—还可能会影响其逃离捕食者的能力—研究员们在实验室中建立了所选鳉鱼的3D模型，它们在怀孕期间如气球般鼓胀的肚皮会让它们的体积增加43%。

They then measured the drag forces the models experience in water flowing at various speeds.

后来他们测量了不同的流速中这个模型受到的水流拖拽阻力。

Unsurprisingly, “swimming” was much harder for the bigger models. 

令人惊讶的是，对于更大的模型来说“游动"难多了。

Drag forces grew exponentially with the 3D models’ volume increases. 

随着3D模型的体积增长，拖拽力成指数级别增长。

This implies the fish pay a price for bearing more offspring, the researchers report today in the Journal of the Royal Society Interface. 

这表明鱼儿为了生育更多的后代付出了代价，研究员们今天在英国皇家学会会刊上发表了结果。

It might seem straightforward that a pregnant, less-streamlined fish suffers stronger drag forces, but this study is the first to quantify how much a pregnant animal’s body changes add to drag forces when swimming.

尽管怀孕的，身体不那么流线型的鱼拖拽力增多好像是很明显的，但这次研究是首次定量研究怀孕的身体是如何在游泳中增加拖拽力的。

But the speed of water flow also matters. 

但水流的流速也是有作用的。

At high speeds, the researchers say, drag forces increase much more rapidly than at lower speeds. 

研究员们称，在急流中拖拽力比在缓流中增加得多得多。

That means pregnant fish may not have it too bad if they slow down and swim at a more leisurely pace. 

也就是说怀孕的鱼如果减速，游得更悠哉的话情况就会好得多。

问题

文中提到了哪两种会影响怀孕的鳉鱼游泳时拖拽力的因素?

A.体积    B.水的颜色    C.流速     D.密度

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