A) 66 B) 67 C) 69 D) 71
Solution: The correct answer is C) 69
$ {{\alpha }_{dc}}=\frac{69}{70} $
$ {{\beta }_{dc}}=\frac{{{\alpha }_{dc}}}{1-{{\alpha }_{dc}}} $
$ \therefore {{\beta }_{dc}}=\frac{\frac{69}{70}}{1-\frac{69}{70}} $
$ {{\beta }_{dc}}=69 $