下列物质中,属于氧化物的是( )
A:O2
B:MnO2
C:KClO3
D:H2CO3
某养鱼专业户为了估计鱼塘中鱼的总条数,他先从鱼塘中捞出100条,将每条鱼作了记号后放回水中,当它们完全混合于鱼群后,再从鱼塘中捞出100条鱼,发现其中带记号的鱼有10条,估计该鱼塘里约有____ 条鱼.
—Why are you standing here, Tom?—I can't see the blackboard clearly. Two tall boys are sitting ____ me.