使用显微镜时，如果光线较暗，为能看到一个明亮的视野，应（ ）
A．缩小光圈，用凹面镜
B．放大光圈，用凹面镜
C．缩小光圈，用凸面镜
D．放大光圈，用凸面镜
显微镜视野亮度的调节方法为：小光圈通过的光线少视野暗，平面镜只能反射光线不能改变光线强弱，因此用小光圈、平面镜调节会使视野较暗；大光圈通过的光线多视野亮，凹面镜使光线汇聚，视野亮度增强．因此若光线太暗，应用大光圈、凹面镜调节，会使视野变亮．
故选：B．
| 选用所给词的适当形式填空，每个词只能用一次。 | |
2. Zhalong is one of the world's most _____wetlands because it _____food and shelter for plants, fish and birds. 3. If we want to learn English better, remember "Practice makes _____." 4. Many rare animals in China are under state _____. 5. If people don't protect environment for wildlife, more animals _____. 6. Look at some pictures of different kinds of birds carefully, then _____ them. |