If $$\sigma_ {s}$$ is the stress in bar and $$\tau_{bd}$$ is the design bond stress, then the development length of a bar of diameter \phi is given by

A

$$\frac{4 \phi \sigma_ {s} }{\tau_{bd}}$$

B

$$\frac{ \phi \sigma_ {s} }{4\tau_{bd}}$$

C

$$\frac{2 \phi \sigma_ {s} }{3\tau_{bd}}$$

D

$$\frac{\phi \sigma_ {s} }{3\tau_{bd}}$$