重为80牛的木箱在大小为20牛的水平拉力F作用下，5秒内沿水平地面匀速前进10米

在关系模型中，若属性A是关系R的主码，则在R的任何元组中，属性A的取值都不允许为空，这种约束称为______规则。

为了采用有效途径向社会公布存在安全隐患的药品信息和药品召回的情况，国家食品药品监督管理局和省、自治区、直辖市药品监督管理部门应当建立什么制度？

患者，男，22岁，发现双眼视物成双半月，劳累时加重，发病前感冒发热病史。检查：血压110／20mmHg，全身一般情况可，眼部情况：视力右1．2，左1．2，双眼前节及眼底未见异常，向前平视，右眼上睑遮盖角膜至瞳孔中央，角膜映光，左眼L／R8°；交替遮盖：右眼由下一正，左眼由上一正；代偿头位：头微向右倾。

为明确诊断，下一步的检查应为（）

A.复像检查

B.行甲状腺功能检查

C.提上睑肌肌力检查

D.新斯的明试验

E.眼眶CT检查

Although the human population around the forestland in Middlesex County has increased, the amount of forestland has not been reduced. Therefore, the decrease in the county’s songbird population cannot be attributed to the growth in the county’s human population.
Which of the following, if true, most seriously weakens the conclusion above

A．As the human population of Middlesex County has grown, there has been an increase in the number of shopping malls built.

B．The presence of more garbage cans resulting from the increase in the county’s human population ensures the survival of more raccoons, which prey on songbird eggs whenever available.

C．There has recently been a decrease in the amount of rainforest land in Central and South America, where songbirds spend the winter months.

D．Although several species of songbirds are disappearing from Middlesex County, these species are far from being endangered.

E．(E) The disappearance of songbirds, which eat insects, often results in increased destruction of trees by insects.