给家兔静脉注射小剂量肾上腺素后，心率加快，心肌收缩力加强，但平均动脉压变化不大，这是
题型：单项选择题
问题：
给家兔静脉注射小剂量肾上腺素后，心率加快，心肌收缩力加强，但平均动脉压变化不大，这是因为肾上腺素（）
A.通过强烈兴奋降压反射
B.通过β受体扩张全身血管
C.通过β受体扩张骨骼肌血管
D.无缩血管效应
E.全身血管收缩
题型：单项选择题
阅读下列短文：从每题所给的A、B、C、D四个选项中，选出最佳选项，将正确的选项涂在答题卡上。
C
California Condor’s Shocking Recovery
California condors are North America’s largest birds, with wind-length of up to 3 meters. In the 1980s, electrical lines an d lead poisoning(铅中毒) nearly drove them to dying out. Now, electric shock training and medical treatment are helping to rescue these big birds.
In the late 1980s, the last few condors were taken from the wild, and there are now more than 150 flying over California and nearby Arizona, Utah and Baja in Mexico.
Electrical lines have been killing them off. “As they go in to rest for the night, they just don’t see the power lines,” says Bruce Rideout of San Diego Zoo. Their wings can bridge the gap between lines, resulting in electrocution(电死) if they touch two lines at once.
So scientists have come up with a shocking idea. Tall poles, placed in large training areas, teach the birds to stay clear of electrical lines by giving them a painful but undeadly electric shock. Before the training was introduced, 66% of set-freed condors died of electrocution. This has now dropped to 18%.
Lead poisonous has proved more difficult to deal with. When condors eat dead bodies of other animals containing lead, they absorb large quantities of lead. This affects their nervous systems and ability to produce baby birds, and can lead to kidney(肾) failures and death. So condors with high levels of lead are sent to Los Angeles Zoo, where they are treated with calcium EDTA, a chemical that removes lead from the blood over several days. This work is starting to pay off. The annual death rate for adult condors has dropped from 38% in 2000 to 5.4% in 2011.
Rideout’s team thinks that the California condors’ average survival time in the wild is now just under eight years. “Although these measures are not effective forever, they are vital for now,” he says. “They are truly good birds that are worth every effort we put into recovering them. ”
According to Paraghaph 5 ,lead poisoning（）.
A. makes condors too nervous to fly（）
B. has little effect on condors’ kidneys
C. can hardly be gotten rid of form condors’ blood
D. makes it different for condors to produce baby birds