单选题 Inland water may be grouped into two general classes: standing waters and flowing waters. As in often the case, the boundary between these two classes is not sharp and clear. A pond(池塘) is an example of standing water. But most ponds are fed by springs or brooks(溪流) and most have an exit. Thus, some current of changing water flows through them. On the most hand, a river is an example of water. In some places, however, a river may have such a slow current that is very difficult to notice.
Standing inland waters contrast in size, in age, and in many abiotic environmental characteristics. They range in size from roadside puddles(水洼) to the Caspian Sea(里海). 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 season. Standing waters range from very shallow to very deep, from clear to dirty, from fresh to salty.
In flowing waters we roughly note the differences 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 limnologists(湖泊学家) in studying flowing-water ecosystems.