小宏同学坐在行驶的汽车里，如果说他是静止的，则所选择的参照物是（）

脊椎动物和无脊椎动物的最大的本质区别是（  ）

    This is a s____  of twin brothers, Justin and Paul. Their friend Susan calls them for h____ . Justin and Paul drive to Hill Building q____ . Three man in police uniform p____  them into a green van. They are not p____ . They are the robbers. The twin brothers aren't a____ . Paul opens the back door of the van w____  a knife. They take different r____  to the police station. They meet the robbers a____  in the police station. They are s____ . The policemen stop the van because the back door of the van is open.

在用氯酸钾和二氧化锰的混合物制取氧气实验后，为了进一步探究二氧化锰的用量对氯酸钾分解速度的影响，小明用如图装置，以及实验室提供的实验所需药品和器材进行实验。老师对实验的建议：①每组实验氯酸钾用量可取4克，二氧化锰质量不宜超过2克。②实验应设计4组。