（1）用向上排气法收集氧气，如何判断氧气已收满？
（2）用排水法收集氧气，如何判断氧气已收满？
（1）向上排气法收集氧气，氧气的密度比空气的大，是从集气瓶底部开始慢慢充满的，若在瓶口能使带火星的木条复燃，则说明已收满，
故答案为：用带火星的木条放在集气瓶口，若复燃，则已收满．
（2）用排水法收集氧气，开始集气瓶中装满水后倒扣在水槽中，产生的氧气将水排除，当在瓶口看到有连续气泡冒出，说明已收满，
故答案为：若在瓶口看到有连续气泡冒出，说明已收满．
| Lichens may grow on the bark of a tree in a steaming tropical rain forest, on the bricks of big city buildings, on rocks in hot springs, on wind-swept mountaintops, and in the driest desserts. In the arctic, they provide the principal food for caribou, and they are one of the few plants that grow in Antarctica. They are pioneers, appearing in barren rocky areas and starting the formation of soil in which mosses, then ferns, and then other plants can take root. Lichens are a partnership of two plants — fungi and algae. The lichen body is made up of a network of fungal strands. In the upper layers of these grow groups of algae. The two organisms live together to the benefit of both, a relationship known as symbiosis. The fungi provide support, absorb water, and shelter the tender algae from direct sunlight. The algae carry on photosynthesis and provide the fungi with food. The algae can live independently and are recognizable as a species that grows alone. The fungi, on the other hand, cannot live apart from their partners. They can be placed in known classes of fungi but are unlike any species that lives independently. So definite are the form, color, and characteristics of these double organisms that for hundreds of years, they were classified as one. More than 15,000 "species" were named. If these organisms are classified as separate species, it is difficult to fit them into the existing system of classification. But if they are classified separately, these species of fungi seem rather strange. Lichens are a splendid example of the difficulties faced by taxonomists in classifying species. |
Which of the following can be inferred about the effect of direct sunlight on lichens
A．It damages the algae.
B．It helps the fungi absorb water.
C．It is required for the algae to carry on photosynthesis.
D．It destroys the fungi.