肺心病继发性红细胞增多是因为（）
A.血液黏度增加
B.急性缺氧
C.慢性缺氧
D.血流阻力增加
E.肺动脉高压
参考答案：C
—Do you know how many gold medals the 23-year-old Michael Phelps _______ at the
2008 Summer Olympic Games?
—Eight. [ ]
A. win
B. wins
C. won
D. has won
在使用union关键字进行集合运算时，保留所有字段的关键字是（）。
A、both
B、double
C、all
D、different
