【单选题】【消耗次数：1】
分子不具有红外活性的，必须是（ ）
①
分子的偶极矩为零
②
分子没有振动
③
非极性分子
④
分子振动时没有偶极矩变化
参考答案：
复制
纠错
相关题目
【判断题】
甲烷和四氯化碳是对称分子，各键矩向量和为零，故为非极性分子。氯甲烷分子中C-Cl键矩未被抵消，μ=1.94D，为极性分子。 所以，键的极性和分子的极性是不相同的。
①
正确
②
错误
随机题目
【单选题】
关于方程的解的表述，正确的是
①
方程的个数小于未知数的个数的齐次线性方程组必无非零解
②
方程个数等于未知数的个数的齐次线性方程组有非零解的充要条件是系数矩阵的行列式不等于零
③
已知齐次线性方程组的两个不同的非零解，则这两个阶的和不是该齐次线性方程组的解
④
已知齐次线性方程组的一个非零解，则该解的任意实数倍也是该方程组的解
【单选题】
行列式<img class=jc-formula data-tex=\left| 1\quad 2\quad 3\\ 0\quad 1\quad 2\\ 0\quad 1\quad 1 \right| src=https://huaweicloudobs.ahjxjy.cn/35F4EB43933DE58FFABB5DE412E3F58F.png style=vertical-align: middle;/>的值为
①
<img class=jc-formula data-tex=-1 src=https://huaweicloudobs.ahjxjy.cn/7D7B521365F25DF6E98C94C0CFE31F90.png style=vertical-align: middle;/>
②
1
③
2
④
3
【判断题】
答题说明：句子没有语法错误请选择“正确”，有语法错误请选择“错误”At the early nineteen century, people did not use stamps.
①
正确
②
错误
【判断题】
答题说明：句子没有语法错误请选择“正确”，有语法错误请选择“错误”There is a various of programs that one can see on every channel.
①
正确
②
错误
【多选题】
On May 6, 1840, post offices___________ England began to sell stamps.
①
above
②
throughout
③
all over
④
across
【单选题】
Television is a remarkable_________ !
①
discussion
②
suggestion
③
invention
④
declaration
【单选题】
You can buy things_________ special low prices when department stores have sales.
①
at
②
in
③
on
④
for
【单选题】
阅读理解：阅读下列短文，回答文后问题，选择最佳答案Trees mainly serve man in three important ways: They provide him with wood and other products; they provide him shade; and they help to prevent droughts and floods.Unfortunately, in many parts of the world, man has not realized that the third of these services is the most important．In his eagerness to make money from trees, he has cut them down in large numbers, only to find that without them he has lost the best friends he had. And besides, he is usually too careless to plant and look after new trees. So the forests slowly disappear.This does not only mean that man will have fewer trees. The results are even more serious: for where there are trees, their roots break up soil allowing the rain to sink in and also bind(固定) the soil, thus preventing it from being washed away easily；but where there are no trees, the rain falls on hard ground and flows away, causing floods and carrying away the rich top-soil. When the top-soil is gone, nothing remains but worthless desert.Two thousand years ago，a rich and powerful country cut down its trees to build warships, with which to gain itself an empire．It set up the empire, but, without its trees, its soil became poor and grew weak. When the empire fell to pieces, the home country found itself faced by floods and starvation(饥荒)．2. In many places forests slowly disappear because __________.
①
A. many trees have been cut down by man
②
B. new trees are not well looked after
③
C. man has not paid enough attention to planting trees
④
D. all the above