下列命题错误的是（ ）
A．命题“若m＞0，则方程x2+x-m=0有实根”的逆否命题为：“若方程x2+x-m=0无实根，则m≤0”
B．“x=1”是“x2-3x+2=0”的充分不必要条件
C．命题“若xy=0则x，y中至少有一个为零”的否定是“若xy≠0，则x，y都不为零”
D．对于命题p：∃x∈R，使得x2+x+1＜0；则¬p是：∀x∈R，均有x2+x+1≥0
由逆否命题的定义，可以得到命题“若m＞0，则方程x2+x-m=0有实根”的逆否命题为：“若方程x2+x-m=0无实根，则m≤0”故A正确；
当x=1时，x2-3x+2=0成立，但x2-3x+2=0时，x=1或x=2，故“x=1”是“x2-3x+2=0”的充分不必要条件，故B正确；
命题“若xy=0则x，y中至少有一个为零”的否定是：“若xy=0，则x，y都不为零”故C错误
命题“∃x∈R，使得x2+x+1＜0”的否定是：∀x∈R，均有x2+x+1≥0故D正确
故选C
| 完形填空。 "I don't have to go to school. My school __1__ me!" Sara showed me her room. "This is my classroom and this is my teacher and the other __2_ " She pointed to a large radio with a microphone. Sara lives with her parents on a farm in Central Australia. It's about 60 miles from her farm to her neighbors. She can't go to __3__ because her farm is so far away from __4__. There are a lot of children like her in Australia. The  y use the "School of the Air". There are 12 of __5__ for children in the outback(内地) of Australia. They are at these schools until they are about 12 years old. Sara said, "We talk to our teachers every day on the __6__. And our __7__ last about 30 minutes. Our teachers __8__ us the class work and we do it before our lessons. Twice a year our teacher__9__ us, and once a year all the students _10__. That's the best time of the year. " | ||||
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