For a T-section, if \(1 < \;\frac{{x_u}}{{{D_f}}} < \;\frac{7}{3}\), the equivalent rectangular stress block for concrete, with uniform stress 0.447fck will have depth of ______.
1
0.15xu + 0.95 Df
2
0.19xu + 0.65 Df
3
0.35xu - 0.65 Df
4
0.15xf + 0.65 Df
5
0.65xf + 0.15 Df