Just like weather, life ______ be beautiful all the time.
A:mustn't
B:needn't
C:can't
D:wouldn't
回答下列问题:
①接触面所受的压力大小;
②接触面的粗糙程度;
③接触面积的大小.
接下来小伟通过如图所受实验操作验证他的猜想:
44×25=____