Interference fringe width is given by $\beta_2 = \lambda \frac{D}{d}$ So, $\beta \propto \frac{1}{d}$ (i) If the coherent sources are infinitely close then the fringe width will be so large that a single fringe will occupy the whole screen. So, no interference pattern will be detected. (ii) If the coherent sources are far apart, then $d$ is very large. So, the fringe width will be so small that those cannot be resolved separately. So, no interference pattern will be detected.