Seven species of carp(鲤鱼) native to Asia have been introduced into United States waters in recent decades, but it's four in particular－bighead, black, grass and silver－that worry ecologists, biologists, fishers and policymakers alike. Introduced in the southeast to help control weeds and parasites in aquaculture (水产养殖) operations, these fish soon spread up the Mississippi River system where they have been crowding out native fish populations not used to competing with such aggressive invaders. The carps' presence in such numbers is also harming water quality and killing off sensitive species.
Asian carp are strong to jump over barriers such as low dams. They lay hundreds of thousands of eggs at a time and spread into new habitat quickly and easily. Also, flooding has helped the fish expand into previously unattainable water bodies. And fishers using young carp as live bait have also helped the fish's spread, as they have boats going through locks up and down the Mississippi.
The federal government considers the Asian carps to be annoying species and encourages and supports “active control” by natural resources management agencies. Federal and state governments have spent millions in tax dollars to prevent the carp from making their way into the Great Lakes, but an underwater electric fence constructed to keep them out has not worked as well as hoped, and policymakers are reviewing other options now.
In the meantime, state and federal agencies are monitoring the Mississippi and its branches for Asian carp and testing various barrier technologies to prevent their further spread. For instance, the National Park Service is cooperating with the state of Minnesota's Department of Natural Resources to construct new dams that are high enough to prevent Asian carp from jumping over. The Asian Carp Regional Coordinating Committee has funded DNA monitoring in potentially affected water bodies whereby researchers can determine whether the troublesome fish are present just by the biological footprints they leave behind. Individuals can do their part by not transporting fish, bait or even water from one water body to another, and by emptying and washing boats before moving them between different water bodies.
Asian carp have been introduced to the USA in order to ________.

A．improve water quality

B．help sensitive species

C．control the number of native fishes

D．control the ecosystem in aquaculture

The measure to keep Asian carp out of the Great Lake but proved not good is ________.

A．testing various barrier technologies

B．construction of an underwater electric fence

C．Emptying and washing boats before using them.

D．monitoring the Mississippi and its branches for Asian carp

It is suggested from the passage that ________.

A．aquaculture operations are dangerous to the water bodies

B．scientific technology is useless in preventing Asian carp spreading

C．both the state and the individuals can make efforts to keep out Asian carp

D．Asian carp have successfully invaded the Mississippi and the Great Lakes