关于静脉瓣的描述哪些正确
A. 为两个半月形薄片
B. 根部与内膜相连
C. 游离缘朝向血流方向
D. 由静脉内膜向管腔折叠而成
E. 可防止血液倒流
下列哪些成分参与微循环的构成
A. 微动脉和微静脉
B. 真毛细血管网
C. 直接通路
D. 动静脉吻合
E. 毛细血管前微动脉和中间微动脉
The battle of Saratoga was very important in American War of Independence, because ___________.
A. it saw the surrender of the British General Cornwallis
B. it is a very important turning-point towards the good of the Americans
C. it was won under the ledership of a French General
D. none of the above
America's Declaration of Independence was worked out in ____.
A. Chicago
B. Philadelphia
C. Washington
D. New York
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