Which of the following hydrocarbons has the lowest dipole moment?

(a) \begin{matrix} CH_3 & & & & & & CH_3 \\ & \diagdown & & & & \diagup & \\ & & C & = & C & & \\ & \diagup & & & & \diagdown & \\ H & & & & & & H \end{matrix} 

(b) CH _3- C \equiv C – CH _3

(c) CH _3 CH _2 CH = CH _2

(d) CH _2= CH – C \equiv CH

[IIT, 2002]