题目内容：

Inland waters may be grouped into two general classes: standing waters and flowing waters. As is often the case, the boundary between these two classes is not sharp and clear. A pond is an example of standing water. But most ponds are fed by springs or brooks and most have an outlet. Thus some current of changing water flows through them. On the other hand, a river is an example of flowing water. In some places, however, a river may have such a slow current that it is very difficult to detect.

      Standing inland waters differ in size, in age, and in many abiotic environmental characteristics. They range in size from roadside puddles to the Caspian Sea. Puddles may last for only a few days or weeks; ponds, for a few hundred to a thousand years. In general, lakes are older, though the waters of some tropical “lakes” disappear completely during each dry season. Standing waters vary from very shallow to very deep, from clear to muddy, from fresh to salty.

      In flowing waters we roughly distinguish between brooks, creeks, and rivers. The size and age of flowing waters are unimportant. Speed of flow, clearness, oxygen content, and other chemical characteristics are used by scientists in studying flowing-water ecosystems.

1.According to the passage, which of the following characteristics of flowing waters is unimportant?

A. Clarity.      B. Size.      C. Speed of flow.      D. Oxygen content.

2.The word “abiotic” in paragraph 2 can best be replaced by ______.

A. nonliving     B. living      C. coastal           D. inland

3.Which of the following statements is true?

A. Usually the water in a river flows into a pond.

B. Scientists only study flowing-water ecosystems.

C. Usually ponds last much longer than puddles.

D. The Caspian Sea is considered as an example of flowing water.

4.From this passage we may know that the distinction between standing waters and flowing waters ______.

A. depends on how people name the waters

B. is clear

C. depends on the seasons

D. is hard to make

5.This passage mainly tells us that ______.

A. there are two major classes of inland waters

B. lakes and rivers are flowing waters

C. age and size of flowing waters are important

D. ponds and brooks are standing waters