y = e^log_ex  =  x^log_ee =  x

Hence dy/dx = d/dx(x)  =  1

Hence 1 should be correct answer..

Its based on pipelining concept which says :

Let the one packet travel from source to destination and thereafter every transmission time interval the next packet will also reach the destination..

So time taken by 1st packet  = 3 * T_t(Data needs to be put into source end, router 1 and router 2) + 3 * T_p (There are 3 links in between)  

                      =  3 * (packet size/bandwidth) + 3 * (link length/velocity of link)

Having found that , as said earlier now at each transmission time interval , one by one 999 packets will be received at destination end..

So for that we need   =  999 T_t

So total time needed  =  3T_p  +  1002 T_t

In 3-D geometry , we have :

(x - x1) / (x2 - x1)  = (y - y1) / (y2 - y1)  = (z - z1) / (z2 - z1)  =  t(say) where t is a parameter..

Hence given coordinates = (1,2,1) and (2,5,8)..

Hence substituting values we get

(x - 1) / 1  = (y - 2) / 3  = (z - 1) / 7  =  t

==>  x = 1 + t ,

        y = 2 + 3t,

        z  = 1 + 7t

Hence B) should be correct answer..

Yes bidirectional means both way implication holds which is nothing but equivalence ..In digital logic the same thing becomes XNOR as an analogy..