题干

昆虫观察盒的盒盖是一个凸透镜(如图所示),盒盖到盒底的距离为10cm,利用凸透镜能成正立、放大像的原理可以方便地观察小昆虫。在一次实验中,小明不小心把盒盖上的凸透镜打碎了。为了修好观察盒,老师分别给他提供了焦距为4cm、8cm、10cm和12cm的凸透镜各一个,他应该选择()

A:焦距为4cm的凸透镜

B:焦距为8cm的凸透镜

C:焦距为10cm的凸透镜

D:焦距为12cm的凸透镜

上一题 下一题 0.0难度 选择题 更新时间:2018-05-09 01:02:49

答案(点此获取答案解析)

D

同类题5

阅读理解

    Winters are long and unforgiving in North Dakota. The winter of 1996 was especially brutal. It was a hard time in my own life too, A neck injury had kept me flat in bed for nearly a year. “Just in time for Easter,” my husband, Dick, said. But how could I feel the joy when the snow was four feet deep and I had months of painful physical treatment ahead?

    I was doing the dishes one day, feeling hopeless when there was a tap against the glass. It was a branch of the troublesome cottonwood (棉白杨).Back in the fall of 1979, it was a new subdivision (分支) then, an eight-foot stick. The people who'd briefly occupied the house before us had placed the pipe from the pump next to it. The earth was so wet that the poor thing had fallen down, most of its bare root system pointing skyward, and blowing hopelessly back and forth in the cold wind. Dick decided to pull it out one day, but I disapproved of it.

    “Look at how hard it's trying!” I said, pointing to the way it strongly kept hold of the earth. “It deserves a chance.”

    Dick borrowed some tools. We packed dry soil around the tree and put up some stakes (桩) into the ground, making it stand upright. That winter was still terrible. Surprisingly, in the spring my “rescue stick” put forth a few leaves, then with lots of branches. The year after that, we were able to remove the stakes. By the 1990s that little stick was a giant, towering over the house.

    Now the tapping at the window continued, louder as the wind picked up, almost as though to tell me to look up. At last, I did. I caught ray breath. In the window against the icy blue sky, thousands and thousands of fresh red buds were waving in the wind.

    The tree was bursting with life and I had a wonderful Easter.