15÷5=____ 14÷7=____ 16÷4=____
30÷6=____ 9×5=____
下列现象中,属于非条件反射的是( )
下列各句中,语言得体的一句是 ( )
--- I've spent the whole afternoon reading books.
--- I think you'd better go out and get some _______ air.
Parents always tell us not to stay ________ there is danger.
方程3x2﹣4x﹣1=0的二次项系数和一次项系数分别为( )