It has been announced that all the candidates______ remain in their seats until all the papers have been collected.

A．can

B．will

C．shall

D．should

慢性肺心病形成肺动脉高压的主要机制：

A．支气管感染

B．红细胞增多

C．血液粘稠度增加

D．肺小动脉痉挛

E．肺毛细血管床减少