如图所示, 一平面简谐波沿 $x$ 轴正向传播, 已知 $P$ 点的振动方程为 $y=A \cos \left(\omega t+\phi_0\right)$, 则波的表达式为
$\text{A.}$ $y=A \cos \left\{\omega[t-(x-l) / u]+\phi_0\right\}$
$\text{B.}$ $y=A \cos \left\{\omega[t-(x / u)]+\phi_0\right\}$
$\text{C.}$ $y=A \cos \omega(t-x / u)$
$\text{D.}$ $y=A \cos \left\{\alpha[t+(x-l) / u]+\phi_0\right\}$