21-三体综合征母亲D/G易位者，再发风险率为1%；父亲D/G易位者，再发风险率为4

I went to Professor Jayne, who listened to my arguments but remained  unmoved. I decided to try harder,

although I didn' t know what that meant because school had always been easy for me. I read the books more

carefully, but got another 77. Again, I       with Professor Jayne. Again, he listened patiently but wouldn' t

change his  mind. [ ]

A. quarreled

B. reasoned

C. bargained

D. chatted

浮髌试验主要是检查（）

A.髌骨有无骨折

B.膝关节滑膜炎

C.膝关节腔积液

D.髌韧带是否受损

E.膝关节活动度

水射流切纸机，以超高速度喷射出的水射流对纸张进行裁切

关系R（A，B，C）和S（C，D，E）进行关系运算，下列各关系表达式成立的是（）。

A．π_A，B(R)π_D，E(S)

B．π_A，B(R)-π_C，D，E(S)

C．π₃(R)∪π₁(S)

D．π₂(R)∩π₁(S)

简述企业债券的特点。