根据下列材料，回答46-50题
Toads are Arthritis and in Pain
Arthritis is an illness that can cause pain and swelling in your bones． Toads, a big problem in the north of Australia, are suffering from painful arthritis in their legs and backbone, a new study has shown． The toads that jump the fastest are more likely to be larger and to have longer legs． (46)．
The large yellow toads, native to South and Central America, were introduced into the north-eastern Australian state of Queensland in 1935 in an attempt to stop beetles and other insects from destroying sugarcane crops． Now up to 200 million of the poisonous toads exist in the country, and they are rapidly spreading through the state of Northern Territory at a rate of up to 60 km a year． The toads can now be found across more than one million square kilometers．(47) A Venezuelan poison virus was tried in the 1990s but had to be abandoned after it was found to also kill native frog species．
The toads have severely affected ecosystems in AustraliA．Animals, and sometimes pets, that eat the toads die immediately from their poison, and the toads themselves eat anything they can fit inside their mouth． (48)
A co-author of the new study, Rick Shine, a professor at the University of Sydney, says that little attention has been given to the problems that toads face． Rick and his colleagues studied nearly 500 toads from Queensland and the Northern Territory and found that those in the latter state were very different． They were active, sprinting down roads and breeding quickly． According to the results of the study, the fastest toads travel nearly one kilometer a night．(49) But speed and strength come at a price -- arthritis of the legs and backbone due to constant pressure placed on them．
In laboratory tests, the researchers found that after about 15 minutes of hopping, arthritic toads would travel less distance with each hop (跳跃)． (50) These toads are so programmed to move, apparently, that even when in pain the toads travelled as fast and as far as the healthy ones, continuing their relentless march across the landscape．
____________
A．But this advantage also has a big drawback -- up to 10% of the biggest toads suffer from arthritis．
B．The task now facing the country is how to remove the toads．
C．But arthritis didn't slow down toads outside the laboratory．
D．Toads with longer legs move faster and travel longer distances while the others are being left behind．
E．Toads are not built to be road runners -- they are built to sit around ponds and wet areas．
F．Furthermore, they soon take over the natural habitats of Australia's native species．
47、____________
A．But this advantage also has a big drawback -- up to 10% of the biggest toads suffer from arthritis．
B．The task now facing the country is how to remove the toads．
C．But arthritis didn't slow down toads outside the laboratory．
D．Toads with longer legs move faster and travel longer distances while the others are being left behind．
E．Toads are not built to be road runners -- they are built to sit around ponds and wet areas．
F．Furthermore, they soon take over the natural habitats of Australia's native species．
48、____________
A．But this advantage also has a big drawback -- up to 10% of the biggest toads suffer from arthritis．
B．The task now facing the country is how to remove the toads．
C．But arthritis didn't slow down toads outside the laboratory．
D．Toads with longer legs move faster and travel longer distances while the others are being left behind．
E．Toads are not built to be road runners -- they are built to sit around ponds and wet areas．
F．Furthermore, they soon take over the natural habitats of Australia's native species．
49、____________
A．But this advantage also has a big drawback -- up to 10% of the biggest toads suffer from arthritis．
B．The task now facing the country is how to remove the toads．
C．But arthritis didn't slow down toads outside the laboratory．
D．Toads with longer legs move faster and travel longer distances while the others are being left behind．
E．Toads are not built to be road runners -- they are built to sit around ponds and wet areas．
F．Furthermore, they soon take over the natural habitats of Australia's native species．
50、____________
A．But this advantage also has a big drawback -- up to 10% of the biggest toads suffer from arthritis．
B．The task now facing the country is how to remove the toads．
C．But arthritis didn't slow down toads outside the laboratory．
D．Toads with longer legs move faster and travel longer distances while the others are being left behind．
E．Toads are not built to be road runners -- they are built to sit around ponds and wet areas．
F．Furthermore, they soon take over the natural habitats of Australia's native species．