A:have produced
B:have been produced
C:are producing
D:are being produced
质点是理想化模型.下列实例中,能把研究对象视为质点的是( )
You are supposed to be more careful to make fewer while you are writing.
This place _______ a factory but now it is a park.
对于抛物线y=-13(x-5)2+3,下列说法正确的是 ( )