下列哪项检查对诊断肥厚型心肌病有重要的意义（）

A.心电图

B.超声心动图

C.胸部X线检查

D.心导管

E.心血管造影

A jar contains only x black balls and y white balls. One ball is drawn randomly from the jar and is not replaced. A second ball is then drawn randomly from the jar. What is the probability that the first ball drawn is black and the second ball drawn is white ？（）

A.A

B.B

C.C

D.D

E.E