What should be the minimum value of b in order that contact is maintained across the entire active face of the gear?

Question

A helical gear has the active face width equal to b pitch p and helix angle . What should be the minimum value of b in order that contact is maintained across the entire active face of the gear?        

(a) p cos $\alpha$                                        (b) psec $\alpha$  

(c) p tan$\alpha$                                        (d) p cot $\alpha$