内共生学说认为线粒体的祖先是（）
A.革兰氏阳性菌
B.革兰氏阴性菌
C.绿藻
D.褐藻
E.黑藻
参考答案：B
把代数式xy2-9x分解因式，结果正确的是（ ）
A．x（y2-9）
B．x（y+3）2
C．x（y+3）（y-3）
D．x（y+9）（y-9）
We didn't know ______, so we gave it to the police.
[ ]
A. whose wallet it is
B. whose wallet was it
C. whose wallet is it
D. whose wallet it was
