搜题请搜索小程序“爱下问搜题”
问题 选择题

三角形的外心是（     ）。

A．三条中线的交点。

B．三个内角的角平分线的交点

C．三条边的垂直平分线的交点。

D．三条高的交点。

答案

C.

题目分析：三角形的外心是三条边的垂直平分线的交点.

故选C.

考点: 三角形的外心.

单项选择题

翻译起始复合物的组成是

A．DNA模板+RNA+RNA聚合酶

B．DNA蛋白+开链DNA

C．核蛋白体+蛋氨酰tRNA+mRNA

D．翻译起始因子+核蛋白体

E．核蛋白体+起始tRNA
查看答案
完形填空

Early one morning, more than a hundred years ago, an American inventor called Elias Howe finally fell asleep. He had been working all night on the design of a sewing machine but he had run into 1.__________ very difficult problem. It seemed impossible to get the thread to run 2.__________ (smooth) around the needle.

Though he was tired, Howe slept badly. He turned and turned. Then he had a dream. He 3.__________ that he had been caught by terrible savages 4.__________ king wanted to kill him and eat him 5.__________ he could build a perfect sewing machine. When he tried to do so, Howe ran into the same problem as before. The thread kept getting caught around the needle. The king flew into the cage and ordered his soldiers to kill Howe. They came up towards him with their spears 6.__________ (raise). But suddenly the inventor noticed something. There was a 7.__________ in the tip of each spear. The inventor awoke from the dream, 8.__________ (realize) that he had just found the answer 9.__________ the problem. Instead of trying to get the thread to run around the needle, he should make 10.__________ run through a small hole in the centre of the needle. This was the simple idea that finally made Howe design and build the first really practical sewing machine.
查看答案