桥基牙的固位作用与哪项无关()A．力的大小 B．桥体的跨度 C．彻底的牙髓治疗 D．

什么是电容器？怎样计算电容器的电容量？

人类胚胎干细胞研究和应用的伦理原则是

A．尊重、知情同意、安全和有效、防止商品化

B．尊重、有益于社会、安全和有效、防止商品化

C．不伤害、知情同意、有益于病人、限制商品化

D．有益于病人、知情同意、安全和有效、防止商品化

E．尊重、知情同意、安全和有效、有益于病人

关于齐多夫定，下列哪点不正确：

A．可减少HIV孕妇感染胎儿的发生率

B．与司他夫定合用可增强疗效

C．抑制骨髓造血功能

D．是目前AIDS的首选药

E．肝功不良者慎用或禁用

在Word编辑状态下，将选定的中文和英文同时设置为不同的字体，应使用______。

A．“格式”菜单下的字体选项

B．“工具”菜单下的“语言”选项

C．“工具”菜单下的“拼写和语法”选项

D．“格式”工具栏中的“字体”下拉列表框

About fifty years ago, plant physiologists set out to grow roots by themselves in solutions in laboratory flasks. The scientists found that the nutrition of isolated roots was quite simple. They required sugar and the usual minerals and vitamins. However, they did not require organic nitrogen compounds. These roots got along free on mineral inorganic nitrogen. Roots are capable of making their own proteins and other organic compounds. These activities by roots require energy, of course. The process of respiration uses sugar to make the high energy compound ATP, which drives the biochemical reactions. Respiration also requires oxygen. Highly active roots require a good deal of oxygen.
The study of isolated roots has provided an understanding of the relationship between shoots and roots in intact plants. The leaves of the shoots provide the roots with sugar and vitamins, and the roots provide the shoots with water and minerals. In addition, roots can provide the shoots withorganic nitrogen compounds. This comes in handy for the growth of buds in the early spring when leaves are not yet functioning. Once leaves begin photosynthesizing, they produce protein, but only mature leaves can "export" protein to the rest of the plant in the form of amino acids.

Roots have the ability to ______.

A．make proteins

B．obtain fresh air

C．produce inorganic nitrogen

D．carry out activities without energy