几个同学围着一台显微镜时，视野变暗，其原因可能是______．_爱下问搜题

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问题填空题

几个同学围着一台显微镜时，视野变暗，其原因可能是______．

答案

显微镜视野中的光线是通过反光镜反射外面的光线进入的，所以若许多人围着显微镜，外面的光线被挡，进入反光镜的光线减少，可能视野会变暗．

故答案为：遮挡进入反光镜的光线

单项选择题

有关窦房结细胞4期自动去极化的离子基础，正确的描述是

A．IK通道进行性失活是自动除极最重要的原因

B．T型Ca²⁺通道激活，这种通道可被维拉帕米阻断

C．Na⁺内流进行性增强，这种电流可被河豚毒阻断

D．L型Ca²⁺通道激活，这种通道受儿茶酚胺调控

完形填空

Every summer, hundreds of thousands of students travel to other countries looking for work and adventure. Most of the opportunities are in   1  work. The pay is usually poor, but most people work   2  for the thrill of travel. You can pick grapes in France, entertain kids on American summer camps, and, of course, there are always   3  in hotels and restaurants.
But it is not as easy as it used to be to find work. Unless you speak the language of the country well, there will be very   4  openings. For example, when you arrive to wash dishes in a restaurant in Paris, the owner will   5  you to speak French. British students only have a language   6  for jobs in the USA and Australia.
Not every one   7  the experience. Sarah James was once responsible for forty American children in Europe. During the   8 , one child lost his passport; four children were lost in Madrid for a whole day; the whole group was thrown out of one hotel because of the  9  they made. Sarah says, “It really was a 24-hour-a-day job since the kids never   10 ! And the pay was awful. It wasn’t worth it.”
The trouble is that   11  expect to have an easy time of it. After all, they see it as a  12 . In practice, though, they have to work hard. At the same time, all vacation work is casual work, and jobs are   13  only when the hotel, the restaurant, or the campsite is busy. But students have few employment   14 . As soon as the holiday season finishes, companies will get rid of them. And if their employer doesn’t like them, they’ll be   15 , too.

C．professional

B．voluntarily

D．continuously

C．interesting

A．experiences