hai vat dao dong dieu hoa tren hai duong thang canh nhau song song ,cung mot VTCB la goc toa do.$x_{1}=3\cos (\frac{5\pi t}{3}+\frac{5}{6})cm, x_{2}=5\cos (\frac{20\pi t}{3}-\frac{2\pi }{3})$ .thoi diem dau tien khoang cach hai vat lon nhat la (tu thoi diem $t=0$)
$A. 0,1 B.0,05 C.0,5 D.2$ (s)