Blue－green algae grow abundantly in salt marshes.

Alligatorsstillexistinsomeofthedank潮湿swampsandbayous小海湾ofAlabama'scoastalregions.

The first important exposition in the United States was held in Philadelphia in 1876.

Communicationisoneofthemostimportantbondsthatholdculturalsystemstogether.

Comets are still regarded with awe by some people.