管束的顺排和叉排是如何影响换热的？

生物遗传信息的中心是______．染色体主要是由______和______组成．______是主要的遗传物质．基因是______片段．______ 控制性状．基因在体细胞中一般是______存在的，在生殖细胞中通常是______存在的．

何谓休克？

Y221封隔器的坐封方式是（）。

A、提放式

B、旋转

C、自封

D、液压

地域主义小说

Paleontologists hypothesize that modern birds evolved from the family of dinosaurs that included Tyrannosaurus rex. This hypothesis would be ply supported if evidence that dinosaurs from this family had a body covering resembling feathers could be found, but so far no such evidence has been found.
Which of the following, if true, would most help the paleontologists explain why no evidence of feathered dinosaurs has yet been found ？（）

A. Fossilized dinosaurs have shown many birdlike characteristics, such as bone structure and winglike arms.

B. If birds are in fact the descendants of dinosaurs, then it can be argued that the dinosaurs never really died out.

C. Flying dinosaurs such as the Pteranodon, which is not thought to have been related to modern birds, do not appear to have had feathers.

D. Soft tissues such as skin and feathers do not fossilize like bones, and therefore are far less likely to have left permanent evidence in the fossil record.

E. The thousands of dinosaur fossils excavated by paleontologists represent only a tiny fraction of the billions of dinosaurs that once lived.