~When using the kinematics equation $x\textbackslash{} =\textbackslash{} x\_{0}+v\_{x0}t+1/2a\_{x}t^{2}$, which of the following is always assumed to be true? 
选项为:A:$a\_{x}\textbackslash{} =\textbackslash{} zero$  B:$x\textbackslash{} \gt\textbackslash{} x\_{0}$  C:$v\_{x0}$~is positive.  D:$a\_{x}$~is constant.  请回答其中一个选项.