2017年北京高考英语完形填空:2017年高考英语完形填空巩固练习：第2套

You might think fish were born swimmers. Not so, however.
　　When they hatch, fish larvae (幼虫) struggle to get around and to　1　. Some 99 percent of all fish mortality (死亡率) 　2　in the larval stage. Only when they grow, do some fish get better at　3　enemies. Adults make quick getaways using a burst-and-coast (爆发并滑行) method in which they　4　a lot of speed quickly and then　5　. Many species don’t have this　6　at birth.
　　Scientists have long　7　that the lack of larval　8　prevented the little ones from gaining　9　force in the burst phase. And perhaps their very small size caused the water to act more like syrup (糖浆) 　10　them, preventing a good coast.
　　11　a new study of zebrafish larvae　12　it’s their poorly devel oped side fins (鳍) and ineffective swim bladder that hold them back. They simply can’t　13　their bodies balanced. The finding might　14　other species, researchers say.
　　“Force can explain　15　of the poor swimming in larvae, but not all, and the difference in coasting ability cannot be explained by　16　in body length either,” said study leader, Ulrike Muller of Wageningen University in the Netherlands. “Many fish larvae hatch without fully　17　pectoral fins and all hatch　18　a swim bladder, so　19　problems could occur for them.”
　　Muller is　20　her findings today at the Society for Experimental Biology’s Annual Main Meeting in Canterbury.

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