28. 已知双曲线C:y2a2−x2b2=1(a>0,b>0)C:\frac{{y}^{2}}{{a}^{2}}-\frac{{x}^{2}}{{b}^{2}}=1(a>0,b>0)C:a2y2−b2x2=1(a>0,b>0)的离心率为2,则其渐近线的倾斜角为((( )))