发文字号的作用是什么？编排发文字号应当注意哪些问题？

当T1指标（），说明在该段时间内，控制区对联络线功率波动有责任，但其责任未超过允许的程度。

A.小于100%

B.大于100%

C.100%~200%

D.大于200%

真性红细胞增多症，下列实验结果可能降低的是：（）

A.中性粒细胞碱性磷酸酶积分

B.红细胞生成素

C.血尿酸

D.血清γ－球蛋白

E.血清维生素B结合力

读4008005006时，读出的零的个数为[ ]

A．1    

B．2   

C．3    

D．4

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.

What is the main topic of the passage

A．The relationship between a plant’s roots and its shoots.

B．What can be learned by growing roots in isolation.

C．How plants can be grown without roots.

D．What elements are necessary for the growth of plants.

(二)

某写字楼工程，地下1层，地上15层，框架一剪力墙结构。首层中厅高12m，施工单位的项目部编制的模板支架施工方案是满堂扣件式钢管脚手架，方案由项目部技术负责人审批后实施。施工中，某工人在中厅高空搭设脚手架时随手将扳手放在脚手架上，脚手架受振动后扳手从上面滑落，顺着楼板预留洞口(平面尺寸0.25m×0.50m)砸到在地下室施工的王姓工人头部。由于王姓工人认为在室内的楼板下作业没有危险，故没有戴安全帽，被砸成重伤。

问 题

何谓“三宝”和“四口”本例的预留洞口应如何防护？