搜题请搜索小程序“爱下问搜题”
问题 选择题

下列各项表达式正确的是

A．N2的电子式：

B．CO2的电子式 :O::C::O:

C．NaCl的电子式：

D．F－ 的结构示意图：

答案

答案：D

题目分析：A不正确，氮气的电子式应该是；B不正确，CO2的电子式应该是；氯化钠是离子化合物，电子式应该是，C不去，所以答案选D。

点评：该题是中等难度的试题，也是高考中的常见题型与重要的考点。该题基础性强，难易适中，主要是考查学生对常见化学用语的熟悉掌握程度。该类试题需要明确的是常见的化学用语主要包括元素符号、化学式、化合价、电子式、原子结构示意图、结构式、结构简式以及方程式和各种模型等，需要学生熟练记住，并能灵活运用。

单项选择题

为了在报表中打印当前时间，这时应该插入一个 ______。

A．表达式控件

B．域控件

C．标签控件

D．文本控件
查看答案
单项选择题

Some spiders hunt on the ground, others build webs to trap their food, but the grass water spider catches its prey by running along the surface of the water.
This special water spider lives on the grassy banks of streams where mosquitoes, damsel flies and other insects come to feed and breed.
Although it is one of the largest spiders in New Zealand, it has an unusual ability. It doesn’t disturb the water as it waits for its meal, and there is barely a ripple when it skims across the surface at lightning speed to catch its prey.
Grass water spiders deal swiftly with larger insects like damsel flies by pulling their heads under the water and holding them there until they drown.
After a meal, the grass water spider spends up to half an hour grooming itself. It wipes its eight eyes, brushes its antennae, and takes special care to clean the hairs on its body.
It is the hairs that trap tiny bubbles of air so that the spider can run down a blade of grass and stay underwater for up to an hour when it is frightened. The hairs also keep the spider dry, even underwater.
It is only when the female spider is caring for the young that she does not hunt on the water. After mating, she produces a large egg sac, which she carries around for five weeks. Once the eggs start to hatch, she attaches the sac to some blades of grass or a thistle. She then tears the sac open and releases the tiny spiders into the nursery web.

The passage tells us that the spider ______.

A. feeds grass and thistles to its young
B. lives on blades of grass under the water
C. lives in the grass on the banks of streams
D. eats a meal once every five weeks
查看答案