The huge Florida wetland known as the Everglades is a slow-moving river 80 kilometres wide but only a few centimeters deep. People call the Everglades a “river of grass” because sawgrass covers most of it. Sawgrass is not really grass. It is a plant that has leaves edged with tiny sharp teeth that can easily cut through clothes—and skin!
Travel in the Everglades is difficult. You cannot walk through shallow water because the sawgrass will cut you. The water is too shallow for regular boats. So, we use an airboat. An airboat is a flat, open boat. Like an airplane, it has a big propeller to move it. The propeller is fixed on the rear of the boat. It makes a tremendous noise, but it does the job. The boat skims along the water’s surface. Although we can still get lost in an airboat, at least we are above the alligators(短吻鳄).
While hundreds of different kinds of animals live in the Everglades, the most famous is surely the alligator. Once endangered, alligators are now protected within Everglades National Park. Visitors are likely to see them both on land and in water.
For a long time, dangers have threatened the Everglades. Around 1900, some people felt this precious wetland should be drained (排干). They said it was just a big swamp and not good for anything. In the 1920s, there was a land boom in Florida. People wanted to build homes everywhere, including in the Everglades. They built canals, levees (防洪堤) , and other water systems that stopped the rivers flowing into the Everglades. Factories were built near rivers that flowed into the wetland. These factories dumped poisonous waste that damaged the Everglades ecosystem.
 People are now working to preserve the Everglades National Park for the future. Right now, one big problem is the paperbark tree. This tree is an invader from Australia.
Paperbark trees soak up a lot of water. In the early 1900s, people brought them to Florida because they thought they would help drain the Everglades. However, the invaders adapted too well. Paperbark trees have taken over hundreds of thousands of acres of the Everglades and killed other trees. Scientists are cutting down these invaders or spraying them with herbicides (除草剂) to kill them. 
Which helps to explain why it is difficult to travel in Everglades?

A．Airboats may make a very big noise.

B．You may get lost when passing through.

C．Paperbark trees soak up too much water there.

D．Many different kinds of animals are to be protected.

Why do people use airboats instead of normal boats?

A．They have big propellers to move them faster than alligators.

B．The propeller makes loud noise so as to scare alligators.

C．Their flat bottom can skim along the water surface.

D．They can watch alligators without hurting them. 

The following measures were taken to drain the Everglades except that people______. 

A．built canals and levees to stop the rivers flowing into Everglades

B．built factories near rivers that flowed into the wetland

C．brought Paperbark to soak up water in Everglades

D．are cutting down these Paperbark trees

The underlined word "invader" probably means something______. 

A．that moves in from another place	B．that enters and takes control
C．that has been brought in	D．that is in danger