搜题请搜索小程序“爱下问搜题”
问题 选择题

如图，一个小木块在斜面上匀速下滑，则小木块受到的力是（　　）

A．重力，弹力、下滑力和摩擦力

B．重力、弹力和下滑力

C．重力、弹力和摩擦力

D．重力、下滑力和摩擦力

答案

小木块受重力、支持力和摩擦力处于平衡状态．故C正确，A、B、D错误．

故选：C

单项选择题

关于遗属收入给付团体寿险和团体信用寿险，下列说法正确的是（ ）。

A．遗属收入给付团体寿险的被保险人是雇员，受益人是其配偶或子女

B．团体信用寿险的投保人是债权人，其负责交付保费

C．团体信用寿险的保险责任只包括死亡责任

D．团体信用寿险保险的被保险人是债权人，受益人也是债权人
查看答案
单项选择题

You can tell the age of a tree by counting its ring. But these records of a tree’s life really say a lot more. Scientists are using tree ring to learn what’s been happening on the sun’s surface for the last ten thousand years. Each ring represents a year of growth. As the tree grows, it adds a layer to its trunk, taking up chemical elements from the air. By looking at elements in the rings from a certain years, scientists can tell what elements were in the air that year. Dr. Stevenson is analyzing one element, carbon-14, in ring from both living and dead tree. Some ring go back almost ten thousand year to the end of the Ice Age. When Stevenson followed the carbon-14 levels changed with the intensity of solar burning. You see, the sun has cycles. Sometimes it bums fiercely, and at other times it is relatively calm. During the sun’s violent periods, it throw off charged particles in fast moving streams called "solar winds". The particularly interfere with the formation of carbon-14 on earth. When there is more solar wind activity, less carbon-14 is produced. Ten thousand years of tree ring show that the carbon-14 level rises and falls about every 420 years. The scientists concluded that solar wind activity must follow the same cycle.

What affects the amount of carbon-14 on earth().

A. The lifecycle of trees.

B. The number of trees.

C. The intensity of solar burning.

D. The quality of air.
查看答案