煤气中毒的原因是因为一氧化碳和血红蛋白结合的能力( )。
A:弱,易分离
B:强,易分离
C:弱,不易分离
D:强,不易分离
地球上最大的生态系统是( )
— I haven't ________ Mike for a long time.
—I think maybe he is too busy to write to you.