椭圆的离心率为()。
A.A
B.B
C.C
D.D
参考答案:B
解析:
由椭圆方程可得:a=2,b=1,所以c2=a2-b2=3,解得[*]所以离心率[*][*]
患者女性,56岁,头晕、心悸3天就诊。心电图如下图所示。
该患者2:1阻滞的部位最可能在()。
A.窦房结
B.房室结
C.结间束
D.希氏束
E.束支水平
Seldom______computer games ever since he entered college.
A.did he play
B.has he played
C.he played
D.he has played